So why not on trail? What kind of surface you are talking about?
locking up the back tire and sliding/grating/dragging it over the trail is about the #1 way to damage a trail while mountain biking. When the tire isn't rolling, the tread is still sliding over the surface, but literally ripping it apart -- this loosens the top surface and really aids in erosion. there has been a lot of debate about the damage that mountain bikes do to tails (many arguing that mountain bike should be outlawed from trail!s). i've seen lots of reports and most show that IF the rear-tire-slide is avoided mountain bikes do LESS damage per distance than hikers or horses or motobikes -- of course mountain bikes tend to log more MILES than hikers or horses... if you ride trails, you can easily see the damage as long marks on the trail surface, usually with a pile of dirt or leaves at the end in the downhill direction... incidentally the 2nd type of damage is shortcuts which hikers also often do as well as trail-widening when riders go around water or mud (stay in the single track and ride through the water or mud unless it is literally not possible or extremely muddy in which case you probably shouldn't be riding the trail).
anyway, NOT locking the rear tire on trails in important b/c:
1) it destroys the trails for others by greatly increasing wear and erosion
2) it really hurts the mountain bike image and our "rights" to use off-road trails (for example Switzerland where officially it is ILLEGAL to use any trail less than 1 meter wide - that's 3 feet! so any single -track is by definition illegal)
interestingly: i learned most of my riding etiquite in the Northwest and learned NEVER to lock up th back brake while riding on trails. I took an "exteme trails" course this summer as part of my "Bike Guide" certification and the Austrian instructor (who was really skilled and could do rear and front single-wheel hops, big wheelie-drops, etc) TAUGHT us to lock the rear brake and slide it in tight corners on single trail --- we had to practice for an hour over and over on this one trail -- i felt so "evil" destroying the trail like that and i won't use that skill ever again on trails
as to the original question: i've never tried locking BOTH wheels and sliding... i think they do moves like that in motorcross, but i think you need pretty high speed and wide tires so probably only works for major "downhill-type" mountain bikes...