I think Gump is absolutely right, some people are just heavy on their kit and will break anything. But basically even the lightest bikes are over-built as far as most riders are concerned because all designers have to spec their tubes to minimise warranty costs or they'll get sacked.
And in fact the tube manufacturers themselves will categorise tubes - e.g. 853 isn't made in very thin gauges, but Reynolds also offer 853 Pro Team which is thinner but intended for road racers - i.e. they can't stop you using it for mtb, but maybe they don't recommend it.
The other thing to remember is that heavier tubes are generally used (a) because they're cheaper and (b) because they're stiffer, not so much because they're less likely to break. A 7-5-7 tube is stiffer and heavier than a 7-4-7, but since most breakages are in the weld area it isn't much less likely to break. Unless you wrap it round a tree of course.
And in fact the tube manufacturers themselves will categorise tubes - e.g. 853 isn't made in very thin gauges, but Reynolds also offer 853 Pro Team which is thinner but intended for road racers - i.e. they can't stop you using it for mtb, but maybe they don't recommend it.
The other thing to remember is that heavier tubes are generally used (a) because they're cheaper and (b) because they're stiffer, not so much because they're less likely to break. A 7-5-7 tube is stiffer and heavier than a 7-4-7, but since most breakages are in the weld area it isn't much less likely to break. Unless you wrap it round a tree of course.
(i know i shouldn't but its just to encouragable) 
