Every aircraft is a compromise, few more so than motorgliders. I regularly shut down my engine on good soaring days, but here in southern Michigan, usable airports and hay fields are *everywhere*. Too, with a Hoffman prop, the feathering mechanism is entirely mechanical and fat blades allow for easy air-starts if the battery gets down far enough the starter won't work. Out west, it's different. A failed restart can be fatal. The more modern TMGs have fully-controllable hydraulic props which is nice in normal circumstances. However, they are, unwisely IMO, set up to feather will loss of oil pressure (like in a twin). The pressure required for un-feathering is stored in an accumulator. They have been known to fail (including during an infamous incident at Sun & Fun with a reporter on board). The engine cannot be restarted in the the feathered position thanks to nanny interlocks. Too, the more modern blade design makes diving no-starter relights both quite iffy and with huge loss of altitude, which may or may not be available.
Flying at near-idle may increase costs but won't get you killed. Costs to soar with a MG are also offset by not having to pay for aerotows at $50+ a pop. Suitable blocking of radiator and oil cooler inlets will keep temperature higher (but still not into the green).
But really, the 'not get killed' is the important element here.
