For games that are established having more platforms to sell on helps, but when getting established sometimes being exclusive helps build publicity.
Infamous vs prototype. Both games were new at the time one was multiplatform one wasn't. While prototype had more at launch over time infamous outsold all the versions of prototype combined. Infamous 2 also outsold prototype 2s combined sales.
But games like final fantasy, wouldn't benefit. Games like tomb raider bayonetta and street fighter probably will sell less being exclusive/timed exclusive will sell less overall than had they been multiplatform from the start but often deals are made that allow the games to be made at all and the salesakes can't really be said to be lost when the game would have sold 0 without it.
