I want a backpack to serve 3 purposes. As a hiking backpack, as a camping backpack, and to serve as a B.O.B.

Now, the top 2 for hiking from research are:

Gossamer Gear Mariposa $130
Granite4 Gear Vapor Trail $150

Any thoughts on using these for B.O.B?

Those look like nice ultralight packs. The only issue I would have is the lack of pockets and maybe the durability of the material. I'd say you can use either one for all 3 purposes. They look a little big for hiking daypack but of course they're very light and you could cinch them down so they don't jostle about. I'm using an Osprey Statos 32 for a daypack and BOB. I've got my stuff a bit modularized so that when I go on dayhikes I can easily remove the BOB stuff I don't need to bring along. Its a little more work using your daypack as your BOB but for me its easier and cheaper than having 2 full seperate kits.

I like me Maxpedition pack... I use it for everything you mention it's def. not an ultra light but it's DEF. rugged and can take a serious beat`n!!!!

I have zero experience with the packs you mention, but wanting one pack to fill three purposes equals a compromise, and I have had just about no luck with those over the years. I fear that you will end up with a pack that will do three things so-so, and one thing not well at all...

A good backpacking pack will also be a good camping and BOB pack, so ignore these last two. Rent, borrow, or buy both, with agreement to return one for a weekend. Leave all tags on, and take each on a hike with all of your backpacking gear. If you can't stand either, return 'em both and start over!

I have found that much of the ultralight gear is strictly for people who make ultralight the raison d'etre of their activities. You have to be careful with it. It just doesn't stand up for rugged or even moderate regular use. I would not want to use it for emergency gear, where reliability is worth saving a few ounces (in a pack). What I mean is, separate ultralight from survival in anything that will take a bit of a beating.

I think this is good advice. I have a fair amount of experience with the ultralight stuff. It is certainly well engineered but it is simply not that durable compared to 1000D Cordura construction. Now, I definitely feel that a pack that weighs 8 lbs empty is going too far the other way; don't get me wrong. But the wispy packs made of SilNylon are just not a good choice for truly bad conditions, IMO.

Everything is a compromise, and as usual in life the extreme ends of the spectrum often cause problems.

For me this would be impossible, but that is based on my definition of a BOB. For me a BOB is something that is supposed to be ready to go at a moments notice & that would provide water & high calorie food for me & my family on the run.

If I was to take it camping with me and eat some of the the peanut butter (now that it is open it will go stale faster), eat some of the food bars, drink the water, use some bandaids, cut up some of the rope, etc. - then I have just reduced the supplies in my BOB.

Now if your definition of a BOB only includes tools (FAK, gun/ammo, knife, rope, snare wire, etc.) then I guess you could make it serve as all three.

If your BOB does include food/water then you will always (and immediately) need to replace whatever is used. But it will take discipline that I must say I don't have. That is why I keep my BOB set aside at home. My BOB weighs 45 lbs so there is no way I would want to carry it everywhere with me, but that is just me.

The pack I carry back & forth to work everday (and take hiking/camping) is a reduced down form of a BOB, but I would want both if truly buggin' out.

I take my BOB camping or on overnight trips, what if something happens when your overnight somewhere and you can't get home for three days, then your bob becomes needed. What I do when I'm home is have my bob handy in the closet and on a shelf above it are enough food and other perishable supplies to re stock it should I use something so that when I return I can drop those in and have it ready to go. Also those supplies are part of my bug in stuff so in addition to the three days supplies in the bob are an additional three or more days right there in the closet.
Then if I do go somewhere for more than one night I'll take some of those supplies along as well since I anticipate on using a few.

I do much the same as Eugene. Maybe it's not a proper BOB, but I have everything I might need in the same cupboard at home. I can pick and choose very quickly, and can see everything at once so nothing will be forgotten. This is in addition to the smaller pre-packed backpacks etc. that I absolutely know I will need and are ready to go (always in the car etc.). I can sort and repack stuff along the way or when I get "there", depending on how it needs to be carried. So my BOB pack is a bit big and strong so that I can just toss stuff in rather than pack it carefully. Yes, it might take me an extra 5 minutes at most, largely choosing and unsecuring a firearm...

I really like the idea of keeping it in the pantry - will start doing that myself. What about other stuff that you use up when camping, etc. ? How much does your BOB weigh when it's fully loaded?

Because packing all the right food for a camping trip usually takes a bit of time, and many times I just want to get out of town on a spur of the moment thing, I keep a Rubbermaid bin filled with camp food that I can just throw in the car at a moments notice. Its like a mobile pantry with all the stuff I normally eat plus some extras. Not as tidy as a fully loaded BOB but I figure (if I'm not taking the car) it would take less than 5 minutes to pack the contents into my large backpack.

I do similar too, when we do go camping then I'll pack things from the kitchen too, but usually items that need more work than whats in my bob. Things like the hotdogs and hamburgers into the cooler. My bob has things like breakfast bars, pop tarts, etc. Stuff that doesn't weigh much, doesn't require cooking, could eat while driving or walking, etc.
I have a similar setup in the garage where I have built shelves along one wall and have some camping gear that isn't in the truck. My ammo boxes and such are there too. Depending on the amount of notice if I were to bug out then I just open the garage door and tailgate and toss stuff in. After we get moved and I build some shelves in our new house I'm going to build one at the same height as the tailgate/bed of the truck and build a small wheeled cart so I can slide them on it and roll it out and slide off into the truck making less trips.
So my bob is part of my whole system, I have my get home bag in the truck, its enough for a night or two stay in the truck or a hotel/motel. Then the bob contains more items for if we would bug out from the house with more food/water/gear to supplement the GHB. Then the camping gear is more longer term. I have a tall cap on the truck so there is enough room we could sleep in it if needed.

IMO
To serve in these capacities I think you will need an internal frame pack.