Dunno if that was an enlightening eye-opener, but if you like him — good for you, I’m all for people having fun with their favourite mercenaries.
No special entry animation, though.
There’s no such thing as ‘meta’ in PvE — mor or less by definition of the ‘meta’. As for productive uses for Leeroy, he’s fun against (heroic) Ahune, alongside the ‘usual’ (at least for me) duo of Velen and Anduin, for example (although there might be potential for failure here, on second thought), with Ninja-Looting for some extra hilarity — by the end of the fight that’ll be some of the biggest Leeroys you’ll see. You could also use him as a finisher (maybe with an Even Bigger Sword) against N’Zoth, alongside Tyrande (with Verdant Recurve to clear out those tentacles), to come in from the bench after a Mindgrasp for a decisive finisher. Come to think of it, he might have some potential against (heroic) Kazakus, although I’ve not tested it… But this discussion probably belongs elsewhere.
As for PvP, I used him a lot, all the way up to top ranks, before the latest big patch, quite successfully, although it’s not what’s been fashionable amongst all those netdeckers. I still beleive, however, that his last ability is potentially problematic, just like the infamous card from the original HS (Leeroy the mercenary, though, is not as irritating, in my opinion).
Apparently, they do monitor forums, although the progress on many issues has been… rather slow. Admittedly, treasures are probably some of the least-used and least-tested parts of the game mode, and thus probably a lower priority as well, but without these reports, they might not even find out that something’s wrong.
A hard cap of some kind is needed, or else there’ll be infinite loops.
Something like the fire team comes in handy when carrying 2-3 mercs for tasks while still doing somewhat meaningful heroic bounties at the same time.
PS As for Moroes — Heroic or not — I see no point in trying to fight him ‘fair’ when you can bring in Yu’lon (the beast equipment would be the Jadefire Spear) with more or less anything else you want, provided that it can reach the boss, and beat the encounter easily.