I can think of arguments against, built on this starting premise:
Premise: We are not an expert site about online games, even if they have roleplaying as part of them. This is pretty much self-evident, but which topics run afoul of it isn't. As a test, we can imagine editing out the details of the specific online game: if the question becomes meaningless, then it would have expected us to have game-specific knowledge that falls outside our scope.
An example of something that passes:
- How to manage the delay caused by text-based communication when roleplaying online.
Something that fails:
- How to organise a RP guild on a WoW RP server.
The first we can help with, the second we can't.
Some considerations that I think show why questions that are inherently about MUDs are off-topic:
- We will never get rules questions related to MUDs from players, since their rule systems are nearly or fully automated.
- The programmers of a new MUD might ask while they're setting it up, but these will either be just about the rules (on-topic) or require us to have knowledge of MUD programming (off-topic). [These are unlikely anyway, since who codes up a MUD in a system they don't understand?]
- MUD-related questions about optimising would either be about the rules (on-topic), or about how to succeed on the MUD (off-topic).
- Non-rules questions (roleplay, group dynamics, time management, narrative tricks, etc.) either will either only mention the MUD in passing (i.e., not integral to the question; on-topic), or will require us to be experts on MUD social dynamics and be off-topic).
This all suggest to me that question that are only about MUDs in passing, where the MUD-ness could be entirely edited out until it looked like just another play-by-chat question, are on-topic. Such question are about roleplaying and roleplaying games (our topic), that just happen to be getting played on a particular digital communications system. If the MUD-ness can't be edited out of the question without damaging its meaning, it's off-topic.