Sure about that?
Suppose mIRC does request the addresses of the existing people in the joined channels.
It means a script has to wait for all received before being sure to have the address available.
Does this make life easier for scripters?

It still can't be done in the same processing cycle.
They still need to split the code up as is the case now.

The most occurring IAL related problem that I see is that scripters try to scram it all together in one code which doesn't work if mIRC has to request the used data from the IRC server.

Suppose mIRC does auto-request it upon own join.
Can we tell them now that they can do that?
No.
Because the chance is there that it isn't present yet.
And they're not gonna accept that it 'sometimes' fails.
I wouldn't too.
What's the gain of this feature then?