This will never get one solid response from the community. We are all divided on the issues of multi-clienting. I'll try to give my insight and non-biased opinions and look at things objectively from what I've seen as both player and owner:
Upside of disallowing dual clienting is the game is definitely more geared towards how it was INTENDED to be played. You were never intended to do everything yourself.
However, as game knowledge improved over the years players just realized they could dual everything. This is not a dual client issue, this is a fundamental game design issue with RO itself. And you can't argue against that. The game is amazing, but it has its flaws and this is amongst the most glaring.
That said, when you actively make the decision to restrict dual clienting you are now ARTIFICIALLY limiting players. They are not actually restricted in game knowledge, you've just put a bandage on a gaping wound. It's a hotfix, but doesn't actually solve the problem.
The problem is the gap between dual clienting and not dual clienting is often extremely massive because of how the game is designed.
I've played servers that are 1x, no dual client and its a blast! its fun to struggle a bit and have to consider other aspects. And for all the people that enjoy that primarily, I get it.
However, in my experience it only servers to limit players artificially when they could otherwise do content. There could be significantly harder content they know how to accomplish if they could just dual client. But they can't.
And despite what ANY server owner will tell you, no population holds up today to justify limiting players in such a way. No dual clients works if you have the players to sustain the alternative. If i can't dual client the content great, but you better have the players to back it up that i can find at will when I want to do said content.
Do any servers have this population? no. not even. close. You will not have a party going on at all times where you wantand when you want. You can form guilds sure, but finding enough people who also have the right classes or are willing to log someone else's character to do said content, is just not feasible.
As such, I'm firmly in the camp that dual-clienting is a necessary evil in the modern era.
In a perfect world, we'd go back to 2004. When the game was at its peak. no dual clients. we all just get to play with each other at our 1x rates and the game sustains itself as it meant to be.
However, the player base is all split up now. and that just doesn't happen.
That said as well I feel its very important that any server takes measures to limit the need for dual clienting at a minimum. If you just take the base game, and leave dual clienting its a pretty miserable experience. Many core necessities simply mean dual clienting. move speed, buffs, warps, etc.
The game works without duals on these things, if you have players. But without players it becomes a suffering experience.
Now, this is also subjective. The game works even without all the buffs, warps etc. you can still play. its just significantly more tedious and grindy. But people forget that its also not necessarily bad either. Most of RO is the journey and not the destination. As such, a longer grind is generally good. But you have to find players who share that mindset, and most dont. Most players are in to get their nostalgia kick and feel like a true end game fantasy is within reach in a reasonable amount of time.
so the TLDR is I feel dual-clienting is a necessary evil, but in a perfect world we wouldn't need it. I won't argue with any server own who feels a need to ban it as my perspective is entirely subjective. I just disagree with the idea that banning duals rather than discourage dual clienting or lessen the need for it is the way to go.