I say NAY! I do love a majority of the legendary Pokémon, yes, but they'll seem less unique with evolutions. After all, if they're one-of-a-kind, and they evolved, then there would have to provide the chance to get another Base-Stage, even if made ridiculously rare. Have you also imagined what their stats would be like? It's quite possible that if a legendary evolved, it would have stats up in the thousands, at least near that anyway. Therefore, it would spawn the overusage of even more legendaries. >_<
Besides, the way that most 1-time Pokémon are designed physically, I highly doubt there's a way to boost their image even more so, especially in the likes of Rayquaza.
Although, I kinda have this idea for an armor suit to attach to Mewtwo, a bit similar to the one he wore in the movie...