Whats the score with the clutch rear mechs, XT in particular, should it be left switched on all the time or will this do any harm? Does it make shifting any harder?

You have to put the clutch in to change gear.

The only time I switch mine off is to remove the wheel.

I turn it off when stored to relieve the spring, but when riding, it’s on all the time.

All of the time for me too.

Mine (SRAM) has no off, can’t see the point in not using it. Another bit of good progress.

Used mine for the first time today. Tried it without the clutch on for the first road section, and then with it on for the rest of the ride. No real noticeable difference in shifting.
Was also running a wide/narrow works ring for the first time today……witchcraft I tell ‘ee! Nary a dropped ring all afternoon, on some proper rooty/rocky trails too.

Only turn it off to remove or replace the wheel.

My XT mech is always left on, and after almost a year of use there’s still plenty of tension in the switch.