The risks and rewards of mob sharing

Lead Systems Designer Greg "Ghostcrawler" Street was on Twitter yesterday, discussing the implications of the planned mob-sharing mechanic being introduced in patch 5.2. If you missed this, MMO-Champion reported on it a few days ago, but the jist of it is that several players can take on rares and quest mobs, regardless of faction, and have a chance at loot without affecting that of the original player attacking the mob. However, the mob's health will increase with every new player attacking it. Ghostcrawler comments as follows:

The concern is that the system will be exploited, as many forum readers have commented, by players who just hit the mob once to get their presence recognized, then leave the original player fighting it to a harder job.

However, on the good sides, if, for example, you're camping a rare spawn that drops a mount, like Aeonaxx, and another player manages to tag him before you do, all is not lost -- you just have to contribute. It's a delicate balance, and what's more it's not wholly new to the game. Certain mobs have behaved like this -- without the health increase -- for a while, such as Problim on Tol Barad, and various others, and while it's possible to tap them and then afk, my experience has been that players want him dead just as much as each other, to complete their quests, and so help out rather than hindering. The health increase only means that the players who want the mob dead will have to work together more effectively.

What this might change is behavior like that which some players report, and which is understandable with how the game works at present. Say you happen upon a player killing a rare, not doing so well. With the current situation you wait and hope they die, so that you can kill it yourself. With the new system, you both benefit from working together. Saying that, if you're doing just fine killing your rare, other players can come along, tag it, increasing it's health, then do as much DPS as a wet haddock, if they don't just stand idly by. And worse, several players could use this to effectively troll the original player, as the health keeps increasing, to, hopefully, a cap.

What's your take on this new system? Do the risks outweigh the rewards?
For clarity, this is different to the new World Boss tap-to-faction system.