Quoting Anthony <wikimail(a)inbox.org>rg>:
Now, if you let the oversighters actually edit the old revisions
freely, they could do what they've done in the past even more
effectively. They could change my edit into a spelling fix, my
summary into "fix spelling", and then change the following person's
edit into a reversion of the spelling fix and the insertion of the
bathroom overflowing statement. But that'd be way way too much power
and ability for abuse. Not to mention unfair to the poor person
making the subsequent edit.
And possibly leading to massive violations of the GFDL. Retroactive
modifications to the history of this sort are just a really bad idea.