The only time you would have to take the -10 accelerated tumbling penalty is if you were a character with a 30ft move, 20ft away from a creature and you moved 20 to it, then had to tumble 10ft more to do whatever you were gonna do (usually for flank, but whatever). Then you would take the penalty becuase if you didn't, you'd not have enough movement to get where you want to.
If you instead moved 20ft, then tumbled 5ft to get inside someone's reach, then you're not taking the accelerated tumble penalty, because you only tumbled at half speed through that area (spending 10ft to go 5ft).
The thief acrobat's class ability lets you tumble at full speed, no matter how many squares you're moving on the grid, with no penalty. Hardly an overpowered ability, because a straight rogue that focuses on tumble can get to +24 pretty quickly, and do pretty much the same thing. Then there are magic items that can help..(+10 Tumble Boots anyone? straight out of the core item creation)