The main reasons that I am against Blank Lands stem from the Blank Lands in Dragon Pass. I was sent a copy of the Gloranthan Encyclopaedia many years ago and that contained some interesting things on the lands of Jarst and Garsting lands to the North West of Balazar. Now, these happen to be Blank Lands, so they will not have anything about them published, even though the information on them was quite interesting. They border on Balazar and there were times when I cold have done with knowing what was there, rather than having to invent things of my own. One of my GMs made Garsting into a medieval land with Knights and sorcery and that worked quite well, but wouldn't fit into normal Glorantha. Then we have the example of Balazar. This land was written up in the Griffin Mountain supplement and expanded on in Griffin Island, but because Griffin Island was made generic, they made Balazar a Blank Land. So, one of the most detailed areas written up was suddenly changed into an area that cannot be written about and about which nothing will be published.
I like Glorantha because it is so well written and detailed. It has lands that have geography, people and places written up and linked together. So, to have areas that are simply holes sitting in well-detailed areas seems a very, very bad idea. Now that we have the YGMV (Your Glorantha May Vary) as a central pillar of the HeroQuest rules, then there seems to me to be no need for Bank Lands. If you want to write about an area of Glorantha, then go ahead. If it disagrees with standard Glorantha then what is the problem? Just blame YGMV.