Reasonable - in general what you fail to consider is that exclusivity is the reason that some games exist in the form that they do. Others have already pointed out that you tend to end up with lowest common denominator games when third parties go multiplatform.
However, exclusivity is even more of an advantage for first party titles. Your original post asks whether or not Nintendo should have to make SSBB a multiplatform title. The real question, though, is whether or not Nintendo would have made SSBB in the same way were it going to be a multiplatform title. As it is, it's a 'system seller'. It's one of the biggest games of the generation and was one of the most anticipated games on the Wii. Would the profit maximizing development strategy have been the same if the game's quality didn't also boost the Wii's mindshare? Wouldn't it have been in Nintendo's interest to not try quite as hard, still assured of selling millions of copies across all systems?
The exclusivity of first party games is the only reason why first parties have an incentive to produce better games than the average developer.
As well, the way things are currently seems just fine. Isn't there near-universal agreement that you can play almost all of the best games of any previous generation with a single console? The market tends to very quickly marginalize all but one of the consoles in a given generation, so savvy consumers need only buy one.
