Slowfall is overwriting tumble completely whether you have slowfall toggled on or off and causing more damage than is normally taken by a build with tumble.
As soon as my Monk got the slowfall ability my tumble skill seemed to no longer help with damage from falls. The falls that used to cause no damage are now causing damage. I tried to toggling it off but then even more damage is taken.
Slowfall is not only 'not useful' at low levels, it is harmful.