I feel like this thread has devolved into just sharing various examples of these pagebreakers. An extreme example of this would be an XSS attack, which could exist (though I doubt it). If one happens to be found, then we would have a problem.
There are also non-toxic uses of interleaved tags, such as this
use of them to make text bigger than the maximum. Perhaps there more useful things like this.
It's probably fine as long as it's contained within the sandbox.
The problem is that there isn't really much to do with it aside from a few tricks (sakaquotes, post-permanent formatting, nested scrollbars) and then new people come in and rediscover everything.
"Build a man a fire and he'll be warm for a day. Set a man on fire and he'll be warm for the rest of his life."