Being able to play without a lot of down time between battles is the primary reason I recommend this pool. Health increases your heal rate and Stamina increases your endurance recovery rate. Together, you won't have to rest between most battles and it's less likely that you'll run out of endurance during a fight.
The other big reason to take Fitness is to augment your travel powers:
Swift
Swift
If you hate running slowly in combat or missions, but don't want to waste endurance on Sprint, Swift gives you roughly the same effect without having to toggle a power.