The argument that "purists" are better than "motorgliderists" is an argument used by miserable old men who try to assert that flying "pure" gliders is more difficult that flying motorised gliders in order to discourage the next generation from adopting emerging technologies.
They do this because they themselves own a "pure" (unmotorised) glider and they don't want their large investment to reduce to $0 as people move away from unmotorised gliders.
It's self-interest, through and through.
Simple as that.

Motorised gliders are far better for several different reasons:

1. Reduce risks associated with landouts
2. Faster turn around time between flights
3. No need for expensive equipment for a launch
4. Reduce the number of people required to launch
......etc....