I think if you're looking at a PC, why would you buy a PC anymore? No really, why would you buy one?"
Full iTunes, the ability to back your photos up to a device with more than 256GB storage and connect an external HDD, being able to manage your music properly and import it from where ever you want such as external media, the luxury of installing and downloading what ever you want, the list goes on...
However, those things are all software related and I do agree the hardware of the iPads is becoming very impressive. So yeah, in a lot of ways what Tim is saying, for me at least is true... Why would we buy a desktop over an iPad when they have become neutered systems such as the iMac and MacBooks where the hardware being used is getting less and less powerful, the premium for slightly more power is outrageous and on top of that Apple have made it damn near impossible to user upgrade.
Apples new objectives should be...
1) Make iOS more powerful if you really want users to replace desktops with iPads as there's very basic things it still can't do.
2) Bring user upgradability back to the Mac, lessen the cost of minimal upgrades and add decent high end upgrades for the same cost so that serious professionals can make use of OS X.