Note that mIRC does poll for your notify list every 30 seconds, but it uses the ISON command, which is much more efficient, and meant for specific users by name.
The issue here is that there is no efficient way to check away status. /WHO #channel might work, but mIRC does not issue /WHO on its own because it can cause other problems (/WHO has been known to flood users off on large channels when done incorrectly). Basically, that command is better left up to the user to deal with. Also, other scripts often need to issue /WHO on their own to update IAL-- if mIRC did this silently, there will be 2 /WHO commands being issued, which is wasteful. If it did not do it silently, it might trip up scripts that rely on WHO responses. What might be nice is for mIRC to fill $nick().away if it sees a proper WHO response, but not issue the command itself. That way users could put /WHO on a timer to get both IAL support and away status scripts working.
Note that other IRC clients might support this feature, but they probably work on a very slow polling rate. If mIRC did this people would complain that it is too slow and ask for it to be sped up, or even not interfere with their /WHO scripts (as mentioned above). Ultimately users would want more control over how things work. This is why mIRC introduced scripting-- so that custom and/or non-standard IRC interactions could be scripted by users in whatever way they wanted, and they would have full control over every detail. This is not possible with builtin functionality, but because many other IRC clients have no plugin/addon/script layer, adding these features as builtin functionality is their only answer. mIRC's answer seems better to me.
Just to clarify and echo what Riamus said, hiding output from /WHO and /WHOIS responses in mIRC is trivial, so that is not an issue. I believe drum offered to show you an example if this is what you want.
