There are airplanes like Citabrias, Decathlons, Cessna Aerobats, Robins and others available for spin training. The main issue I have with gliders being used for power pilot training is the lack of a throttle, I often find during spin training pilots miss setting idle during recovery. It’s highly likely an unintentional spin will be in a situation with power applied.
It’s one thing to verbalise it during a briefing, but the proof is in the application. I usually arm the speed brake during stalling sequences in jet simulator training, following the recovery manoeuvre it’s often still armed. Stowing the speed brake is part of the standard Boeing stall recovery manoeuvre. The trainees might have recited the actions during the briefing or immediately prior to the exercise but often omit the step during the practice sequence. As a human factors consideration I fear power pilots would omit setting idle power if spin training was only conducted in a glider.