Well, a lot of the changes coming in the Core are going to shape the way this game is played. A lot of the reason for disallowing use of multi-logging will become a problem of the past. No more equipment loss means that now the Uber difficulty can be ramped up and infinite-town portal creations will be effortless regardless.
Also, in terms of experience or item drops, I don't think that there's sufficient enough problems with either of those issues to warrant concern.
With respect to your plight, you have to understand that you're battling the concept of a thin white-line of "cheating" to a group of players that have played this Mod since it's birth. We don't extend to the idea that you're incapable of finding a way to level quickly and efficiently without the use of multi-instances - because we ourselves have the effort to do without. I think more than you know, the rules are set in place to protect users. Much like Heathen said in his first post and without inclusion of many other reasons, it's to protect a community of users that don't want to see inflation, multi-logged 8 player runs of 120 Paladins, etc. etc.
So, if you wanted a defined argument for a reason against it, it's simply because as a community - or for most of the players I know - we just don't want it.
I can honestly say with how lazy I am, regardless of the circumstance, I don't want to continually login to servers mass populated by one player. Just remember, multi-logging is a ban-able offense in the original D2 - if anyone remembers, you had to literally open a game, wait 5-7 minutes and then drop your shit and hope it didn't disappear - so, I'm personally thankful that we're allowed to mule safely. Above and beyond that, be thankful there's still an online community able to give you dedicated reasons why NOT to allow this feature and quit arguing it. You're not the first.