Clients/scripts that rely on the account info, for example for recognizing clients or enforcing bans currently are forced to use 'WHO #chan %na' or 'WHO nick1,nick2,..,nickN n%na' to get the account info. And keep doing that at an interval in case the user gets an account set later.
Most useful information from WHO doesnt change (user, ip, server, realname), or when it changes, it is announce to clients (nick change, host change when user is +rx), and only needs to be requested once and then saved for use. But to catch a user getting an account set, one is forced to keep using WHO at an interval.
There is a way to make it easier on such clients/scripts and reduce load/traffic use on the server from WHO, announce to clients when an account is set on a user in a common channel. That way, one needs to do WHO upon join of the user to get some info, and if the user has no account set at that point, it wont matter, as it will be announced to us when it does.
This could be done for example by sending ':<server> ACCOUNT <nick> <account>' to clients on a common channel. Obviously, not everyone would want to receive such account messages, so some sort of setting needs to be there that indicates a client wants to receive these, a (local) usermode, or some other setting to tell the IRCd.