Why would I choose a mastery so my red or blue buff can last 18 seconds longer rather than having 2% bonus damage when isolated or even the ability to heal more from pots???
What's an extra 18 seconds gonna do for me? Can someone explain why they think I should choose this when jungling?
It counts for Baron and Elder too.
If I take pots when jungling I either go 
or at least
rendering the pot buff mastery useless. This is a huge personal preference thing.
2% bonus damage when an enemy is isolated can be a nice little buff to you or generally useless. I take it on assassins/champion I know will be trying to catch people out/invading a lot. Also 2% more damage is pretty garbo on a tank jungler.
I take runic affinity because 18 seconds is enough to refill your mana bar to full, clear scuttle, gank a lane with the last bit of red, clear a camp, etc.
EDIT: plus, like Frost said, it counts for Baron/Elder too. This can be pretty huge late game.
