Touch and goes are a double edged sword. On the good side, they get a lot more approaches and touchdowns in the same amount of side, which saves money. On the bad side, they don't exercise anything in the landing beyond touchdown (and most landing accidents occur due to the pilot not maintaining control after touchdown), they require the pilot to divert attention from aircraft control to reconfiguration for takeoff (again, encouraging loss of control and, in retractables, a potential inadvertant gear retraction on the runway), and they kill any real opportunity for the instructor and trainee to discuss the approach and landing just completed (because the trainee is now busy doing the takeoff, climbout, and pattern). On the other hand, doing full stop/taxi back avoids all those issues, but does take more time and therefore gets fewer landings per dollar for the trainee. On the third hand, those fewer landings may be providing better training quality overall.
So, it's not a cut-and-dried issue, and depends on what you're flying, where you're flying, and how the trainee is progressing -- every case is different, and the answer may change as the trainee progresses.