I agree with Robert to some extent, it's not necessary to learn how to "wheel " without lockers. But I will suggest that having lockers does make it more difficult to learn to drive with finesse, because with open diffs there's a much finer line between success and failure. And while I agree that there is a skill involved in driving with lockers, it is a skill that is easy to acquire if you are competent with open diffs, whereas a driver who has never wheeled with open diffs will find it very hard to get used to their subtleties.
Regarding trail damage, newbies or idjuts can wreck a trail with or without lockers - they'll just get further in with lockers. The same goes for aggressive tyres - with experienced and measured use, aggressive tyres should do less damage to a trail than a tyre that is more likely to spin. But in hooligan or novice hands, they have the potential to do far more damage.
Incidentally, while moving rocks etc. might be considered unsporting, unless it's a contest of some sort, it's a very valid off-roading technique, IMO. As long as it's reasonably in keeping with the nature of the trail, and isn't destructive, then repairing bridges, shoring up embankments, moving fallen trees etc. help to improve the trail!