Slowing an aircraft down is only a by-product of the true function of flaps, which is to give you the same effect as if the wing area were increased (both increasing lift and drag at the same time).

As far as flaps on trainers go... I built a Senior Telemaster and added flaps to it, and nicely "over"-powered with an O.S. 61FSR.

Wow, did that add a load of fun to flying. With the added lift (and drag at the same time) I occasionally like to fly backwards which is very easy to do with a Telemaster in a steady headwind.
When everyone else quits flying because the winds rise to about 15 knots, I crank up the Telemaster and have some real fun. Imagine having a radio control kite. That's what a Telemaster is with flaps.
I've actually landed it like a helicopter where I keep it flying at the same position relative to the ground, and just slowly let it settle onto it's wheels with a "0" rollout. Conversely, I've taken off in a steady headwind to where the take-off roll could be measured in inches before taking to the air.

My Telemaster serves as a "boring" trainer for teaching the new guys at the field, but the flaps make it a really fun airplane that even experienced pilots like to play with.

Don't ever be afraid to put flaps on a trainer. They're totally neutral if you don't deploy them (as if there weren't installed at all), and when you become more confident as a pilot, you can start dialing in just a little at a time and enjoy the wonders of what flaps can do for any airplane.