Revisiting quest encounters for fun

In WoW there are many cases of mobs that only appear as part of a quest. Once you've done that quest, you can never fight that mob again.

Now, for some mobs that's fair enough, that's the way the story works. For some mobs though I'd like to come back and do it again. I'm especially thinking of the various gladiator ring mobs, such as in Nagrand, Grizzly Hills, and Zul'drak. There's also a really fun quest in Storm Peaks, where you jump from flying dragon to flying dragon. Again, you only get to do this once.

Why would I want to fight them again, especially if there's no further quest xp or loot to be gained? Well, these mobs are usually extra tough with an interesting gimmick, and usually need a group to take down ... which makes them great for testing my solo skills later. (This I know because there were a few group quest mobs I didn't get done back at the appropriate level because I couldn't find a group.)

So, two basic reasons then: (1) gauging my increased skill/gear against a known constant, and (2) fun (remember that?)

Why then are these encounters designed to be once only? (I suspect the reason is more that they haven't been designed to be revisited, rather than being deliberately restricted). If the xp and the loot drops from them are kept low (the quests being the main reward) they wouldn't be farmed or exploited.

What might be the downsides to making these encounters able to be revisited?