> And it appears that the cargo sits directly on the roof of the vehicle

Let me talk about that. I had a roof-top carrier that did indeed sit directly on the roof of my car. I recommend against that. It collects water when it rains, which seeps up into the carrier, and it abrades the roof something awful.

My understanding of a roof rack is that it us supported on structural components of the chassis and does not rest on the roof. There may be rails, or the rack may attach too and rest on the gutters, but the load is not supported on the roof itself. The metal (or whatever car roofs are made of these days) is not intended to support heavy loads. (I used my carrier for sleeping bags (how I found out water seeps up into the carrier) and for dirty laundry (yeah, I found out about seepage that way, too, but I didn't sleep in my dirty laundry).)

> Are there some decent, sturdy racks out there, maybe with their own metal
> mesh rack affixed to the frame that would hold a useful amount of weight?

That's what I would look for - a rack or support system that attaches to the frame and which has a basket for the load that keeps the load on the structural parts of the vehicle. "Useful amount of weight" is in the eye of the beholder. I have seen several cars along the side of the road on the way to Burning Man which had tipped over and spilled the contents of their roof rack into the bar ditch. I suspect a combination of speed and swerving to miss a cow or rabbit. Heavy loads on the roof definitely cause stability problems. You may not want a useful amount of weight on top of some vehicles that are reported to be tippy. Be very careful.

I have quite a few roof racks for my vehicles, and like anything else, there are different racks for different purposes, it really depends on what you're carrying and how much you're want to spend.

What you are probably seeing on the road are just factory roof racks which are just like you said, basically a fence to keep the stuff on the roof and give you a place to strap things to. IMO, they're useful for a quick trip to Home depot or to strap something on short term, but more for looks than utility.

Then there are the cargo baskets by Thule, Yakima, and many other which use the factory roof rails. I keep a small yakima basket on my daily driver because it's relatively unobtrusive and I rarely have a need for it, but I like it there just in case. Most of the time I'll use it for stuff that's large and bulky, or if I'm transporting things I don't want in my car (i.e. leaky trash bags, fuel, or other smelly items, etc). It's small enough that I don't notice any drop in fuel economy or stability issues, but since it uses the factory rails it is only as strong as the factory rack.

A couple of examples of a cargo basket

Yakima Basket
Rola Basket
Thule Basket

At the upper end are the heavy duty roof racks that you'll see on a lot of off road vehicles. INTI makes a really good roof rack of this style, or the (now discontinued) Con Ferr is probably the most common I've seen. These racks are very large and heavy (75lb-100lbs or more), putting them on is at least a two person job. These racks will hold anything you can put on top of it, including a couple of guys standing on it while loading. I rarely use mine because you can really feel the weight up there while driving, your gas mileage drops by quite a bit, and personally I just don't like having that much stuff on the roof. But when I need to carry a lot of stuff over long distances, that's the rack I use. I have exceeded the factory weight limit many times, though I think that's more of a legal disclaimer than actual rating. A vehicles roof should be able to support the entire weight of the car if loaded properly, but the stability while driving will suffer even with a minor load up there.

Examples of a heavy duty roof rack
INTI roof rack
Con-Ferr roof rack

One reason I do not like to use roof racks is there is really no way to secure anything up there short of having a cargo box, and even then those boxes aren't that secure. Plus you really have to be careful about low overhangs and parking. structures. I only use it if I'm absolutely crammed full inside the car, and only then I'll try and put the bulky, non-important items up there.

I prefer my receiver hitch-mounted cargo rack to putting a roof rack on my Jeep. My Jeep spends a lot of time offroad where I don't want anything on top of it. I guess if I spent more time on pavement than offroad that wouldn't be such an issue, and I'd be okay with a roof rack so long as everything was securely sealed for protection from getting wet.

I dont know, I kinda like having my roof rack. I dont have any pictures of it loaded down, but when I need the extra room that a Jeep does not afford, it comes in handy. I have put as others have said more bulky stuff up there. It can hold a few hundred pounds of extra gear. Yes it makes it top heavy but if you know that going into the trip, then you know not to take corners like you are in a sports car. All in all it is a handy tool for extra tools.
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I use a Rola rack that's mounted to the existing rails on my Subaru Outback, leave it on all the time as it's a bit of a pain to take on and off and even more of a pain to store it.

It affects my gas mileage a bit but I can live with that.

Very useful as it allows me to shift stuff from the back of the car to the roof when I have passengers, I've also got some very bright back up lights mounted onto it. I've made myself a dual 5 gallon gas can carrier and a full sized spare tire carrier that attach to it that are used when needed as well.

I keep a bag of bungie straps and ratchet straps in the car all the time for attaching things to the rack.

I've thought about getting one of the hitch receiver type of trays but I don't really want to have a hitch on the car. The Subaru's main issue with offroading is it's approach angles, a hitch would make that even worse plus I have to have access to the cargo area in the back all the time and the hitch type of tray would interfere with that.