I read the new article with interest. Like other's have said, they're trying to solve problems I don't have. For example, I learned a long time ago that multiple monsters makes combat more fun for me and my players. This was 3.0. After reading this article, I can't help but wonder if Mike Mearls and I are playing the same game. That said, I'm not worried.
For one thing, if 4e is designed to easily accommodate large groups of monsters, great. Less work for me on that part of prep.
One thing that I haven't seen mentioned in the designer blogs and on D&D Insider is that adding class levels to monsters is probably used more by DMs more than players. I don't add fighter/barbarian/sorcerer /etc... levels to monsters because I want to play them, but because I want tougher or more interesting monsters.
It doesn't matter to me what game mechanism I use, HD advancement, PC class levels, NPC class levels, or whatever, so long as I can scale and customize the monsters. Now, I'll admit, I run a lot of monsters out of the book, but I also customize a lot of existing monsters. Orcs with class levels are not uncommon foes at my table. I doubt I'm in the minority.
Everything I've read about 4e suggests to me that that's going to be possible, but they're going to do it differently. 3.0's problem, monster-wise is that the DM had to reverse engineer the many monster's to customize them. 3.5 acknowledge that and gave a lot more information on building monster's from scratch. If 4e does uses a different game mechanic to achieve the same result, I'll have to see it before I can judge.