Exclusives always tend to be either the most ambitious or the least ambitious of projects (one can see this most clearly on the Wii, i think. Going from heavily ambitious titles like MadWorld and The Conduit down to fanservice like Umbrella Chronicles or Castlevania, and then down again to Ninjabread Man and Anubis II) Its titles of mid-level ambition that seem to go multiplat. The games that are real moneymakers, but also require a lot of input too, like Call of Duty or Rock Band, or even GTA or Resident Evil 5
Exclusives always wind up being those ambitious projects that are really remembered, though. Guitar Hero had its humble beginnings as an exclusive, or the heavily ambitious MGS4. 3rd-party exclusives tend to produce some of the best stuff in this industry. 1st-party exclusives, of course, form the key bullet points between consoles, especially between the PS3 and 360, or between SNES and Genesis, the other competitors that were, sans exclusives, pretty much even. 1st-party exclusives provide the foundation for riskier platforms, too. Like how Halo kicked off the Xbox, or Wii Sports showed the world that Nintendo was relevant again. PlayStation 2 really didn't start with strong 1st-party exclusives because it didn't need to.
In short: exclusives good.
"They aren't obviously. Theres little difference between buying and owning a studio and buying some exclusives."
This is very much false. If you own a studio wholly, you're stuck with them in the short-run no matter what kind of garbage they put out (in the long run you can always close them down or sell them off, but for now you're stuck with what they've got), thus you're financially liable for their screw-ups. Whereas you can choose which 3rd-party exclusives to buy (or which exclusives to sponsor, publish, or distribute), and then guarantee that what you're picking up is the best work possible, and if they turn sour, pull the plug on the whole thing much quicker.