I've opened up 4 centre diffs. The centre diff has two weaknesses.
First. The 4 brass thrust washers. One for each of the 4 gears inside the diff. In perfect condition they keep slack out of the mechanism. The washer sits between the casing and the rotating gear, stopping steel to steel contact. Washers are sacrificial. Washers wear over time. It is easier and much cheaper to replace a washer that is out of specification than worn gears and diff casing. Add to this some reports of poor case machining from factory.
Washers wear more quickly when lubrication is restricted through build up of sediment around them, or when oil levels are low. Each washer may wear at different rates. One may be missing altogether, another with sections missing, one cigarette paper thin and one only half worn - in the one diff.
When they wear away completely or fall apart in the final stages (bits of brass in your oil change) you then have a situation of steel gear running on steel case. If you never stress your drive train ie don't go off road, then you might run almost 'indefinitely' with no problems.
Take a vehicle with some hundreds of thousands of kms, with less than routine servicing of the transfer case. The centre diff is now 'loose' or out of specification because of worn thrust washers and slightly worn pins and gear centre holes. The gears not longer hold at an unflinching 90 degrees to each other. There may be varying degrees of friction on each gear, depending on if you are now missing any thrust washers.
Add to this the second problem. The two separate pins that hold the gears in place. The pins have to cross each other but remain on the same plane. To do this the pins are machined in the middle by half their shaft diameter ie only half their strength in the centre. The pins also have to hold the gears ridgedly at 90 degrees to each other, but they don't. There is a little bit of give. That is why Ashcroft sell a one piece replacement for the two separate LRover pins.
My feeling is that under stress the brass washers do not fuse under heat and jam the diff. They simply fall apart under pressure and rapid rotation, even when new, but particularly if already very worn. All this play then adds to the movement of the cross pins which then break in their centres where they have been machined to half their diameter. Locking the diff gets around the slackness or play.
On tarmac, I think you could drive all day with the diff locked. But you would have tyre wear and lots of vehicle vibration as the axles relieved them selves by forcing the tyre to lose traction by skidding. I drove an old Toyota like that after my friend took it for a test drive but did not disengage 4x4. Yes, you might break some other part of the drive train, but it would be along time coming under these conditions. So brief moments on hard pack between lots of soft stuff won't hurt.
On good gravel roads. I think not going to hurt if locked or unlocked. Differentiation unlocked will generally be within the units design range. Depends on whether you like the understeer.
On high speed corrugations, same as above. If locked, understeer probably not so obvious. On corrugations all wheels are momentarily losing traction to relieve any wind up, and wind up is unlikely as the time interval between crests is not sufficient to allow appreciable increase in rotation of one wheel against all others.
Off road, or generally good gravel but with say lots sand drifts or patches of mud, I would drive locked.
Again, Ashcroft make the solid pin centre, but you'll still have to maintain your thrust washers. They also make the auto torque biasing diff that can take some power away from the wheel losing traction - is good in changing road conditions without the need to be constantly locking and unlocking - but that can be locked in the really difficult stuff. Locking distributes power 50:50, but is also there to protect the Rover diff itself.
