You know, I can give you an economist's answer on the one hand, and on the other, there are pros and cons to doing this. The pro of doing it is that it doesn't induce people to lock their savings into a given asset; it allows them to freely change assets without penalty. The con is that we're taxing capital gains favourably already by allowing you to hold capital gains until you've realized them and, in the meantime, accumulate returns as they accrue.
On purely equity grounds, what we'd like to do is tax capital gains as they accrue rather than when you realize them. Given that we don't have that system, I can see the argument for allowing a rollover provision, but I'm not convinced that....