I often blend red wines when I get a bottle that is too one-dimensional or missing something, I can typically determine another wine I have on hand that would be complementary such that blend is superior to either separately. I've heard people complain that is sacrilege, but those people don't understand that some of the best wines in the worlds go through exactly that same blending process before bottling.
My beer palate & experience is nowhere near as advanced as my wine palate, so I don't think that I could do that very well with beers yet. But I definitely understand the concept behind why someone may do it with beer.
You know, I've never thought to blend wines like that, but after you described it, it sounds like a no-brainer. I don't drink enough wine at home where I often find myself with multiple bottles open, but I might experiment just for the hell of it.
As far as beer goes, I don't generally blend my beer outside of the occasional black and tan. I only keep 3 beers on tap, so if I brew something that's meh It will just end up being the next tap to get replaced when I'm ready to put a new beer on tap. If you keep a bigger stockpile around, I could see how it could be useful, but if I have a good beer on tap I'm a lot more likely to drink it as-is rather than use it to try to save a different batch.