If by some bizarre event one or both of those clients gained a massive portion of the IRC user clientele then many servers would probably be forced to put restrictions on /who, either by ignoring more than x requests per user per hour, or even putting a vey low tolerance on what was considered 'flooding' when using /who. They might even have to limit the number of people using those particular clients, perhaps block them entirely. Ultimately this would mean that those clients would either have to drop or severely limit support of this feature to being practically useless, or otherwise face their users being banned from the majority of IRC servers.
That is an over-exaggeration. I'm 100% sure that no server will ever ban someone for excess use of the /who command. If there was an excess load on the server, some servers might add a feature that would tell everyone in a channel when people went away if they had a certain umode turned on. But you can't deny the evolution of mIRC because some servers might get more use.
