I'd love to see some data on how many people buy a console due to exclusive titles. I've done it, but it's always been Nintendo. (My dog's name is Link so you can guess why.)
The top 3 Xbox 360 games are exclusive (Halo 3, Gears of War 2, Gears of War) and 6 of the top 10. Xbox has the 'Return of the King' exclusive franchise with Halo.
The only company with a massive list of successful exclusives is Nintendo.
You are right about Halo, but it is Microsoft's only RoK exclusive franchise so far IMO. I.e. if both Microsoft and Sony were to launch a new system with a new iteration of one of their best franchises, Halo is the only weapon in Microsoft's arsenal that would enable them to sell more systems than Sony. Gears of War and Mass Effect are great, but they won't individually beat MGS, God of War, Uncharted or Gran Turismo.
During the previous generation of consoles, exclusives were a huge reason why the PS2 ended up on top I think. Its line-up in 2001 alone was incredible: Gran Turismo 3, GTA 3 , MGS2, Twisted Metal: Black, Devil May Cry, Onimusha Warlords, etc.
The top 3 Xbox 360 games are exclusive (Halo 3, Gears of War 2, Gears of War) and 6 of the top 10. Xbox has the 'Return of the King' exclusive franchise with Halo.
The only company with a massive list of successful exclusives is Nintendo.