Those interactions are nice. And they exist in modern games. Just not in instances. You can train people in most mmos if you bring aggro mobs by them and then they leash or join in. I know WoW/WAR/FFXI/TOR did this for a fact. It's one of the reasons people avoid others in open world environments to a degree, because there are a lot of fuckheads on the internet. Except instead of all the fuckhead mmo players being crammed into one game (regardless of playstyle preferences et al) you have lots of games where fuckheads congregate. So people tend to play games where they can avoid having to deal with fuckheads as much as possible.
Those moments are memorable and I will totally agree with it. They also created the desire for people to only form guild groups and to do the most dangerous shit possible so that the average fuckhead trainer would not be around. By creating an environment where fuckheads can proliferate and be their fuckheaded selves willy nilly, you will be creating an environment that most sane people will want to avoid. And since, unlike when we all played EQ, alternatives exist these days, people will leave the game en masse or avoid areas where those fuckheads congregate. Which defeats the "purpose" of having fuckheads in your game since people will avoid them by either leaving their area or leaving the game, since alternatives exist.
Look, I totally get the need to be able to interact with other players. If I could positively affect literally every player on my server on a daily basis, I would be fucking stoked. I have no idea how many random stranger's bodies I pulled up out of shitty situations when I played EQ, for zero reward and only because I knew I could and thus I did. The number tops the amount of times I enjoyed the "leashing" aspect at least 100 to 1. Because while I agree with Khane entirely that leashing should exist logically, I also disagree with the idea that the leashing aspect created unintended bonuses that other games haven't/can't replicate. You can totally build that sense of community (and it was, prior to LFD and increasingly smaller raid sizes in WoW) in modern games, but it is the desire of some people to fuck shit up for other people and be total fuckheads that makes me abhorrent to the idea. I'm no white knight, and I don't want a game of only positives. But I -do- want to limit the amount of fuckheadery that I and others have to deal with on a daily basis from other players. Because if my memory serves me correctly, some faggot guild on EQMac just woke up the sleeper as a big Fuck You to a server that existed fine prior to them joining it. That shit right there? Might make for a memory. But it will definitely make for people being pissed over someone being a fuckhead without recompense.
Unless the game you are designing requires a valid social security number as well as a valid home billing address and phone number to be accessible from in order to log into the game. Information that people you fuck with can see so they can dole out the badly needed ass-whooping that some people really should have gotten when they were younger so they could not be fuckheads. I'm ok with that then. Battle.net ID+10~
Seriously though, again, leashing and its effects were completely unitended and you cannot reproduce that type of environment in today's market because there are options and people know how shit works. It is a great example of how something worked a decade ago. It is not a great example of how things should work today. Unless your aim is to attract only complete mmo newbs and let them experience late 1999 all over again, the intended result will not be the actual result.