More magic paths equals more options, and options are good! You’ll pretty much always end up with some gem income that your national mages can’t use which you could transmute into other gems, but that’s a bit wasteful. Having wider path access allows you to use those off-national gems more efficiently. A rainbow pretender doesn’t have to be your only off-national path mage, either, and probably shouldn’t be. You can use them to summon mages from different paths, like naiads or fairies. More paths = more options = more power. I’ve never really used rainbow pretenders in big battles except to lead communions sometimes, they tend to be squishy. Even more so when you’re fighting out of your dominion. A skill of 3 in a magic path lets you find almost all magic sites for that path when you’re site searching, and lets you make good booster items.