Maybe the mechanic wants to renew the packing while he's at it.
Seen plenty of installations where pulling the engine is the only way to address that unless you're a midget with three left hands and extremely long double-jointed arms.
It may well be easier to remove the trans with the engine, I would leave that to the discretion of a knowledgeable mechanic who is familiar with your rig.
Besides, you did want to repaint that part of the bilge this winter, right?
And touch up those rust spots on the oil pan, and check the bolts on the motor mounts, and a whole bunch of other things that won't happen with the engine in place.