I'd say it depends on first what you have a passion to do and second, what can you afford? If you're pinching pennies, of course you want the one body, at a bargain price, that will do everything you need.
But you might reach a point -- a point of reckoning, in which you say, hey, I could buy this camera body for this but hold onto this camera for that...well, then you're screwed, I mean, what shall we say...you nave joined the club of the passionate and don't want or need to pinch pennies.
So, I have a 30D, which has served me well, I do mostly outdoor photography and as much wildlife as I can, but I wanted to excell in landscape photography as well, so I bought a 5D. Now I'm trying out my 5D in wildlife photography...am I a bad photo-person? I haven't used my 30D in a while, but I'm not rushed, because I'm not counting nickels and dimes...
Follow your passions and your needs, as you can afford! Many of us have more than one body, but that doesn't make a rule for you!
Tony