What the 2/3*s lack in raw stats and skills they at least partially make up for in easily unlocked equip slots, lower costs to evolve (both coins and using lower tier dragons so fewer runs through the 80 stamina dragon maps) and more easily leveled skills. While they will eventually be replaced, they're not a bad investment per se early on.

Personally, if you're going to evolve 2/3*s, I highly recommend giving priority to the ones with high speed, since this is the one stat a 2* can beat a 6* in, and some maps have some wacky speed gap requirements to 3-medal them. After speed comes type (if you're lacking a specific one).