I have all three systems but I mainly play one. It is nice for it's exclusives however - I don't know what I'd do if I didn't play Demon's Souls. It's more than one factor I guess, but the only thing you'll really be missing is a few exclusives from the other system. But there are enough games on both system to keep yourself occupied for a long, long time :)
While people have a primary system I would support multiple system to support multiple ideals. Companies have different approaches to systems.
I tend to approach a set of new consoles with the sense that I will only purchase one. Ive only had a single console for each generation. I might change that eventually and get a ps3,
but in the mean time im enjoying my 360 (really im enjoying my new pc). If I had to choose one electronic medium for video games it would still be the personal computer though
" Well common sense says "monopolies are bad yo". I mean look at the board game, it always SEEMS like a good idea, but noone ever actually has fun playing it. "I have fun playing it. MMUAHAHAHAHAHAHAHA because im so eeevvviiiilllll :P
Is that question directed at developers or consumers?
For consumers, I'd say one console is enough, given that that console isn't a Wii. The Wii isn't a bad thing to have, but if you have nothing but the Wii, you're gonna miss out on just about every game that's worth playing.
I use my 360 primarily for multi-platform/exclusives and my other consoles/PC for exclusives. But if your money is limited, I can see having one console with 20 games being a better solution then owning all the consoles and 2 games for each. So to answer your question, it depends on your situation I suppose.