The claim system definitely needs changes but I'm not sure what the best way to go is. Allowing you to get SP off any mobs regardless of it being claimed would result in XI style parties where you round up dozens of mobs and kill them for insane SP. We don't want that in FFXIV, especially when mob density is higher in this game. I'm also totally against allowing multiple parties to engage a mob.
They need to fix the exploit where a party can keep a mob claimed even though the whole party has died as it's becoming a problem with NMs. I can see it getting worse in the future if it isn't fixed. When the party fighting a mob loses hate the monster should go unclaimed, not reset the rage timer and give you another 30 minutes to try again. Mobs should also be able to draw you in if you try to hold them outside their territory.