Do yachtsmen turn on the motor when under sail to increase their speed a little bit just for fun? Not unless they have to get somewhere in a hurry, or to get somewhere when the wind dies. Kind of defeats the point of sailing. Soaring is simply sailing turned from a 2 dimensional art to a 3 dimensional art, by rotating the lift vector created by moving air to a vertical axis from a horizontal one.
You have to understand that the whole point of soaring is the "game" you play with the atmosphere. You're either into the game or you aren't and a motor pollutes the game if you use it while playing, except in specific circumstances.
Most motor gliders use the motor to launch and hunt for initial lift, then once lift is found, the motor goes off and the game starts (if you're are being towed and your tow pilot is any good, he/she is also hunting for lift for you while on tow and will try to get you near or in a thermal when you get to release altitude).
In general, you will use the motor mainly to launch and find lift, then only to rescue yourself if you get caught low. So the motor is mostly to eliminate the tug, and the need for retrieve crews if you land out, or retrieve tows if you land out on a usable airstrip. High performance motor gliders with retractable motors have to go through a retraction/extension procedure as well, so that is another factor in discouraging motor use until you really need it. Of course, the other problem is you're more likely to need it, because the ballast of the launch motor hurts your sink rate, like having water ballast you can't get rid of, and it's more of a struggle to stay aloft in weak lift. But again, this is normally a late in the day gotta-get-back situation.
On the other hand, there are "touring" motorgliders, with medium gliding performance and that are meant to be usable as long-range cross country aircraft in the travelling sense, with engines and props that facilitate starts and stops quickly at any time, as well as pure soaring, and landing gears that make normal power plane airport operations feasible. You could really call these "hybrid" aircraft.
In these cases you may well indeed use the motor to supplement weak lift if it's to help you get from A to B. But even there, you're making a lot of noise, and it's so much more pleasant with it off, so you'll avoid running the motor when you can. This is probably more akin to "motorsailing" yachts that are a hybrid of power boat/sail boat.
With the newest electric motorgliders, you may use the motor more to help with thermaling, but they still make a bit of a racket compared to motor off (fast turning prop) and have limited endurance, so you still would tend to avoid using it unless necessary.