QUOTE(4EverLost @ Dec 30 2011, 12:14)

there should be a lot of lower levels that have credits to train those but level very fast. If you are high level the credits it takes to train them isn't very hard to get and well worth the price. I'm sure I was in the low 100's when I got them
Well to me at least, it's not about not having the credits for Pack Rat, but rather not needing its final levels - though it is indeed expensive to max it out for someone who's in the 100s and doesn't want to burn all the newly acquired money (RL, IPU, FSM), it's generally the chance to get into auctions and grab some better equip.
I'm at 2/5 Pack Rat atm, and the closest I got to use all my pots was my first Heroic clear vs FSM - used 8/10 iirc.
If one's aiming for the highest difficulty clears though it's kinda necessary to have 13 slots, but on lower levels longer BT/IWBTH arenas are simply out of your leage, or too time consuming to actually be worth the effort, imho.
I'd much rather spend my credits on Adept Learner and Assimilator, since they're pretty cheap and give a big boost to leveling.