t is recommended to select TCAS off while on ground (and not on a runway) because otherwise mode A and C will also be active and lots of returns from many aircraft on ground in close proximity creates problems for ATC secondary radar.
Yes, and in effect, uses up bandwidth which is limited. Currently, ATC has to null out the ac on the ground, but the bandwidth is still being used up. With increasing ground ops managing surface flow, it will become critical to turn systems off.