I would even go so far as to say that in some ways, successful multiplatform titles are better for an ecosystem than are successful exclusive titles.
The advantage of an exclusive title is obvious: it's a safe, reliable bet to appear on your platform, by definition. Naughty Dog's games aren't going to Microsoft any time soon. Nintend EAD isn't making a PS3 game, ever. Further, every system sold on the backs of exclusives will be one of your systems. Madden sells 360s, but it also sells PS3s and even Wii Us; by contrast, Halo sells Xboxes and Xboxes only.
Those are big benefits. The downside to exclusives, however, is that there simply aren't a lot of them anymore, and in particular, most major third parties aren't interested in making them. Activsion, EA, Ubisoft, Take 2; all of these companies' bread and butter games are multiplatform.
So having your system provide the top selling SKU for multiplatform titles is a pretty big deal. It convinces those publishers to keep making games on your system, and to think of them as Xbox titles first and foremost. Call of Duty is on PS3 and PC -- and even started as PC exclusive -- but I think of it as primarily an Xbox game.
Microsoft has the faith of the major publishers, because in recent years the Xbox platform has been very good to them. This constant influx of major multiplatform games provides a reliable and consistent amount of good games for Xbox gamers to play, simply because there are so many more games that fit this description.
Just to repeat: I'm not saying exclusives don't matter. Exclusives have their upsides. But in this day and age, having the top selling multiplatform SKU has its upsides too.