The private server community has always been about different ideologies clashing and coexisting with each other. The beauty of RO is how well servers can be a reflection of their owner(s)'s personal preferences. Of course, if someone doesn't agree with the type of gameplay experience one server provides, it's extremely easy to try your hand at another one. Or, in some cases, find the effort and motivation to build your own server.
All this to say, I don't see what calling multiclient stupid does for anyone. Servers that want to disallow a gameplay system have every right to do so. There is a significant playerbase that enjoys multi-client being banned, and that is why the no multi-client servers have been successful. In the same vein, historically servers that allowed multi-client have been very successful as well. There are people who play RO for lots of different reasons. Some people are very strong minded about a particular feature and that is make or break for them - and others seem to be totally indifferent about it.
On the topic of multi-client moderation being imperfect, therefore it's stupid to enforce: this doesn't make any logical sense. To use an analogy, Windows isn't perfect at catching viruses. Does this mean that their efforts to make operating systems resilient to viruses is in vain? Of course not, it means they have to keep improving their code and whatnot. A server that bans multi-clienters may not be perfect, but I'm sure as competent moderation staff accumulate more and more skill at detecting users violating rules, they'll grow more adept at dealing with them.
Progress takes time and effort. I can't speak on all private servers, but Oath does, in my opinion, have some of the most professional staff I've ever experienced on any private server. I don't play there much personally (I like 3x - 10x rates), but If there was any staff I'd trust to enforce their own rules proficiently and impartially, it would absolutely be them.
So no, banning multi-client isn't stupid. It's the flavor of RO that some servers want to provide, and it happens to be a tasty flavor to a lot of players currently. The owners should be praised for sticking to their vision and working hard to uphold the standards of their server. If enough people disagree with the trends of the most populated servers of the low rates, then now is the perfect time to rally dissenting opinions into action and make a new server that bucks those trends.