I don't think there's a right answer for this, it really depends on your particular situation, where you live, how often you go out, what are your chances actually needing one, etc. In some circumstances, it could be an absolute necessity, in others, it's just a extra weight.

In my case, I don't really have to worry about really extreme cold or a lot of rain, so not having a bivy would not be a life or death situation, it's more of a comfort/discomfort issue. I guess I'm lucky, I can sacrifice comfort for weight. But your situation could be different. You just have to decide whether it worth the weight to carry it around and probably never use. Just think, for the same weight of the bivy, you could be carrying an ultralight down sleeping bag (1 lb) + a couple of trash bags, which I think would be a much better combo. Of course, for a few ounces more, you could have a sleeping bag/bivy combo, or for another few ounces, a small tent, it never ends. At some point, all this stuff stops becoming emergency gear and you're just carrying luxury items to make you more comfortable.

LED,
My poncho isn't that awesome, it's my first time sewing, so it actually looks like a kitten swallowed a ball of thread and threw up on it. I was looking for something that would replace my bivy for an emergency shelter when hiking, and the silponcho looked perfect. Plus it was basically a big rectangle, so I figured it would be an easy thing to sew, there are a lot of plans on the internet. It came out alright, it's functional, so I'm happy with it. I haven't had a chance to try it outdoors yet, so I might still need to modify it. I'm going to see if it works as a vapor barrier liner in case I need to extend the range of my sleeping bag.

WildWeasel,

How do you like the thermolite bivy? I was looking at getting one of those, but I really wasn't sure whether that would be any better than a plastic bag or tarp.