The problem is that oldbies are usually faster and stronger than newbies, hence they will more often be able to claim a monster as their own. I think using some sharing-of-exp depending on the hits scored would at least leave newbies with a slight chance - specially when they tear a rex down and someone comes by to kill the last %.
As for RPing, sure, such a system isn't very rpish, but is killstealing? Well, certainly, it could be, but in 99% of the cases, it's just some powergamer mad for exp stealing a kill from someone who's really working hard to get a foothold. No RPing there.

Ya, they could log each person that hit's a monster as well as the percentage of health they depleted; Then divide the EXP among that percentage.

Item priority would only last 10 seconds, regardless. Also, you wouldn't be able to renew your ownership over some items, because you would wind up picking them up. :o

RPing doesn't happen when you have people with immense cosmic powers who are bored. They just want to kill people.

*enters underground dungeon*
*hits everything once*
*people must pay me cash for me to kill one beast, so they can kill one theirselves*

PWNED

True. Still I think a shared exp system would rather improve rping than the block-system we have now. The shared way, you don't need a party to share exp, but everyone who participates gets their share. It's more realistic for the one who weakens it to get the most exp, while the one scoring the kill should get something, too, than if the weakener gets nothing at all.

You would have to hit everything within 5-10 seconds (5 probably the best idea for preventing abuse since you don't wander while you're killing something), and I don't think someone could get around to all the monsters in the dungeon and maintain ownership over all of them. Eventually you'd accedently kill it, forget a monster, or just get bored and leave. :o