For what i can tell you, Im one of the mechanics at Hobby town and I usually use glowplugs with idle bars if I think the sleeve is on its last legs. It does slightly raise the compression a bit, and for a tired motor it usually helps it out with the little extra compression. I wouldn't use it on a daily basis.

-Davidinmi- Basicly what i was saying above. Rasing the compression in the motor will make the motor run a little bit hotter, and in cold weather, sometimes you need the little extra heat.

Im just speaking from my experience with nitro motors, not no glowplug expert