It's a nice perk for early adopters when the pickings are slim. Once the next generation hits full swing it'll be an afterthought.
I would expect a company to try and incorporate backwards compatibility if at all possible when designing a new console, but in the specific case of the PS4 it appears to be an unfortunate but necessary byproduct of actually listening to developers and what they want in next gen hardware for once - a very positive position to take overall.
Moving away from the cell towards a PC oriented x86 CPU pretty much guaranteed incompatibility with the current library, in the traditional sense at least. If their streaming tech does what they say it does, they'll leverage it to take advantage of their legacy software in multiple ways I'm sure. Having a truly developer friendly console with forward thinking architecture is worth making a clean break in my opinion though, the potential benefits far outweigh the drawback of having to keep a PS3 around to play PS3 games.