I don't need a knob to control the drummer tempo. With the drummer that I play with now (Christian Teele, who we're all very proud of out here in Colorado because he just got back from the Grammies where he jammed with Al Jarreau and DeeDee Bridgewater as the CD he made with them was up for a Children's Grammy. You can also hear him on E-Town, but I digress), I don't even have to look in his direction. By the end of a note with a tempo adjustment, he's locked in.
If you have drummer tempo issues, the only knob that will work would be on the end of a big stick, with which to whack him to wake him up and remind him that listening is more important than playing.
By the way, all this tempo/drummer stuff works great with my Alembicized Starfire!