Once upon a time, in a play-by-post game, we fought a huge executioner with an axe, and his "daughter" (a nasty little skeleton). When the nasty skeleton finally died, the executioner went nuts and started swinging his axe in big arcs, hitting everything around him. I guess you had to be there, but my point is that you don't see many instances of creature abilities keying off other creature's actions.
Which is quite understandable, of course. Monster Manuals, and even monster-themed books like Open Grave are written to be generic, and having monsters give bonuses to other specific monsters goes right against that. At most, we'll get things like the Azer Beastlord, which gives bonuses to "beasts". That's still a huge group, and a thematic pairing with a "beastlord".
However, for the aspiring homebrewer, it's a good thing to keep in mind. Take, for example, a big nasty that gets even nastier when its minions die. That might upset the usual tactic (kill the normal monsters first, since that's a faster way of eliminating a source of damage) and make it a good idea to focus on the leader (so you don't have to deal with him when boosted).
(Note that the "usual tactic" is dramatically appropriate - the BBEG dies last.)
In summary - giving monsters interacting abilities makes them more interesting and can shape the combat.