Hammerhead
Explorer
Negative Zero said:... in addition, who says that sneak attack is the only reason to tumble? flanking already has its own benefits. in addition, maneuverability is often its own benfit as well!
~NegZ
Flanking does provide benefits, but generally a +2 bonus to hit isn't worth the possibility of an extra attack of opportunity headed your way. And often moving to flank means that you lose a full attack, or the option for rapid striking or attacking with both hands.