A computer is constantly upgrading. PS4 might be as powerful as the most powerful computer or, solely for the sake of argument, let's say it's better. A year down the road, do you think it will still be as good as the best computer? How about 5 years? Sooner or later, unless Sony comes up with a way to keep upgrading the graphical powers of their machine, it will become outdated. This is true with pretty much every console.
And while you can argue that Sony is making sure the PS4 can perform multiple functions other than play video games, in that respect it will NEVER be like a PC. You can't use programs on the PS4 that they don't specifically allow you to use, so you'd never be able to get the type of software you'd like to use (like, for example, the Adobe suite or Microsoft Office). The PS4 is for entertainment, pure and simple, and comparing it to a PC is just ridiculous.