You are correct, if a console has similar CPU & GPU as a PC, and you're driving at the same resolution, and set PC graphics to that of the console, then the console will be most likely offer better framerate due to game optimised OS. However that ends within the next gen of PC hardware. It does not cost a lot to improve on the PS3 graphics. Picking up a Nvidia 8800GT for example for £25 now will blow away the PS3 and Xbox 360. Wheras with a console you're lumped with the hardware. And the limitations of it. ie I cannot use a Logitech G25 with the Xbox 360.
Forza 3 just looks terrible compared to the PC, I look over it but it could do with some AA. GT5 in parts looks better thanks to some AA and higher resolution, but I think it looks worse than Forza 3 as it just lacks graphical detail.
And once you're using a higher resolution display, in which case current consoles are topping out at 720p with no or little anti-aliasing, low or none anti-stropic, the PC will pull ahead.
And of course you have backwards compatibility. I can still play the old games. Wing Commander, Lucasart, Abe's Oddysee, Space Quest and other Sierra games, Another World etc.
I have both consoles, they're ok but PC does a lot more. Once you factor in prices for console games, compared to PC, console games are about £10-£15 higher. So if you buy a lot of games, the systems start to be similar price. A gaming rig does not cost a lot more than a newly released console. Also if you have a PC already that saves rebuying parts.
Also PC offers vsync. Consoles sometimes do not, and you get screen tearing with it. You can see this in GT5, or due to hardware not able to push 60fps at a steady rate.
I only have consoles because of the console exclusives. Whenever it's multi-platform it's always best on the PC. Look at Skyrim, for example. HD texture packs, extra spells and weapons, quests, buildings, scenery, pretty much anything.
And unlike the Xbox, on a PC you get free internet play. No need for MS gold subscription (or PS3 +)
And checkout Morrowind, a re-texture pack has been released to improve the old textures.