Because of the slow rate at which you earn Cantina credits, I tend to think of it as a long term investment anyway since either way you'd have to farm 80 shards to unlock either Daka or Fett (unless you're lucky enough to unlock them from a Bronzium like so many others on these forums seem to do :'( ).
Both Daka and Fett are useful at higher levels.
Daka obviously because she's a healer and you can never have enough healers in your roster (especially for GW), but also because of her ability to resurrect a fallen ally and her stun which procs against me way more often than I'd like :(
Fett, like you said, packs a punch and his AoE with a chance to lock down the whole enemy team's abilities is also extremely useful (and so bloody annoying!)
You can tell I'm not bitter at all about fighting against those two characters :p