Rainbow Mages Worth It?

It's up to your playstyle. If you can handle sinking the points and playing with the type of playstyle that means playing with a Rainbow go ahead.

In competitive MP, you should shy away from them except for extreme circumstances (superfast research, superlow independants, or superhigh Magic Sites)

Otherwise you are waiting too late in the game to be effective and unless you get left alone, you will fall to the first one you go to war with.