Lately I've been playing around with this mechanic even though it's never really vital. It's a lot more straightforward to just plan on brute forcing down each and every event, but nonetheless I've found this amusing lately.
Today I was in queensdale leveling a fresh (sylvari) warrior and champ boar and the wasp queen popped at the same time (someone else started the boar event a few minutes earlier, not me). The queen made her way all the way over to the boar and did lots and lots of damage to it, and the boar in turn did lots of damage to her. this mechanic now.
Another application I found is luring the rotting oakheart to the ettins to the east. Even though the ettins are non-vets, they will deal substantial damage if you get lots attacking the champ oakheart.
A third application is some of those gimmicky change-into-some-other-form hearts where you have to fight in some awkward form instead of actually playing your character (if it's not clear enough, I hate these). I lured one of those monsters over to a scale (which regenerate), and the scale made short work of it for me while I watched :-). I would surmise mobs attacking each other function with different formulas, since the "power level" between say, a vet and a normal mob doesn't seem to be that huge between them, whereas as players there's a very obvious difference between norms/vets/champs.
I would guess these cases are designed this way intentionally. -_- The troll in queensdale is just a hop skip and a jump from a vet spider, next time I'm there I might try getting the spider in on the action instead of calling other players, just for shiggles. Anyway I hate seeing 8 people show up only for 2-4 of them to constantly get downed.
In AC there is a troll that will sometimes spawn before the first boss. You can lure the boss down to the troll and the troll will kill the boss. As far as I know you can still do this (I haven't done AC in months) and there are lots of other things like this too.
In WvW EB the Speldon camp, you can lure a oakheart champ into the camp and it will take out the npcs guarding the camp. Of course to me its easier to just solo the camp, but I've seen the oakheart pulled into the camp by people several times, guess they like the show.