Extended operation at idle in the air

This might be an ignorant question, but why can't you just shut it down if it's not needed for extended period?

Not guaranteed to restart :)  looking for thermals, not within gliding range of a landable airport ...

Oh ok... Is anything ever guaranteed in a glider?  :)

Extended idle is generally not good for the gearbox... if it were me, I think I'd rather shut it down, barring some history of starting difficulty.

Thanks wasn't really thinking about gearbox

RE:  Is anything ever guaranteed in a glider? YES 27:1 glide ratio comes in very handy when you are at 2000 AGL and 6 nm downwind of an airport and a spark plug electrode jams a valve

Besides fouling the plugs and dirtying the Oil, you are wasting engine hours.
The engine will need an inspection every 100 Engine-Hours.
If you run the engine at power for 30 minutes per flight with 2 hours of unneeded idling in the middle, you will run through the 100 hours 5x faster than if you were to shut it down.

Engine-Hours are Operating-Hours, without regard to the power setting.
You do not get to discount the idle minutes.

2.5-Hour flights with 30 minutes of engine time each means the airframe will get to 100 hours after 40 flights but the engine doesn't reach 100 hours until the 200th flight.
If you fly less than 100 hours a year, the inspection cost saving might be moot, but the lifetime TBO saving will not!