The problem with this is that the rulebook would quickly become the core book by default. That's ok for those of us who enjoy playing in Glorantha but would mean fewer copies sold for Mongoose.
That said, there is a problem. The campaign Dara Happa stirs draws material from 5 different books as well the core books and that is really not very friendly. There isn't an easy way to deal with it but it would be good to have a one-stop Glorantha book that DOESN'T have the core rules in in order to make room for a sort of Player's Guide Deluxe.
So I vote no. I would like, however, a revised version of the core books at some point because they are a rather rambling mess.