You know, I sometimes see other players getting mobbed by enemies in the outdoor areas. I often check their levels, their health status, see how many criminals are attacking them and how well they're managing. Most times, I leave them be. Other times the player appears to be in trouble and I jump in to assist. Especially when playing as my Tanker, who can draw aggro off the other player and ease the pressure on them.
This, in my mind, isn't kill-stealing. It's doing what heroes do: namely, helping others in need!
I find myself in the reverse situation sometimes, again particularly with my Tanker. I don't really "grind" as such on outdoor foes, but if I see civilians in dire straits I'll help them out because, again, it's what heroes do. And sometimes, thanks to a Tanker's abilities, I find myself getting mobbed by rapidly respawning foes.
When that happens, and I begin to be overwhelmed through sheer numbers, and another hero leaps in to help out? I'm always grateful. We heroes are outnumbered a hundred to one! We need to watch each others' backs!
Given the emphasis on cooperation in CoX and the various measures the game has in place to prevent players from getting shafted too horribly when someone, er, assists in dispatching a foe, getting angry about kill-stealing seems a bit contrary. The only real scenario I could envision being cause for frustration would be a high-level character nuking low-level spawns in order to deny them to other players, but quest areas are generally large enough that a single player couldn't hope to cover everything.