They do exclusives to promote the platform in general, not just to draw in PC gamers. I goes for PS3 owners, Wii owners, people not currently owning a PC or console, etc. Additionally, exclusives may be able to make better use of the hardware, but I'd say it doesn't matter all that much.
However, these days I wonder why we're after bleeding-edge high-tech games so much. I was always a fan of new engines when they came out (I remember my anticipation for releases like Doom, Quake 1, Unreal, Quake 3, Far Cry, Doom 3, Unreal Engine 3, etc.), but what I see on every platform these days is just awesomeness. For all the nostalgia and people going on about and replaying the classics, the emphasis on graphics and detail confuses me.
So PC is currently ahead of the couple-years-old consoles again, not surprisingly. But there is no platform out there that is not capable of producing excellent gaming experiences like we had 5 to 10 years ago with the classics. I see no limitation that has to take away from that.
Maybe the focus of console developers is a bit different, mainly because of differences in the target market. But I see that line fading. Most people enthusiastic about games probably have more than one console and a gaming PC to boot. Incidentally, my PC is a couple of years old and my Xbox 360 cranks out bigger, prettier and more detailed worlds. I still enjoy games on the PC, though.