I'm not really into the block figures myself, but, I can see the appeal. When I was a lad, some of my first coustoms were a result of trying to make a spiderman lego-man.
In the '80s and '90s (I assume when we all grew up) the lego people really hadn't gotten into licencing as much as they have today. The appeal is this as I understand it;
1.) block figures are "hip" and cute, so like, when your ladyfriends comes over, they don't see "action figure" they see "hobby" or quirky accesory. In one of my former employees offices, the head web designer had a big display of customs, all in the block style. Clients come in regularly, and they see this guy's desk and think nothing of it.
2.) The flat surface and bright colors make them super easy and fun to make customs out of, I imagine they are good practice.
3.) Cheaper. ou get 4 to a pack, and the block marvel universe is almost as expansive as the original Toy Biz line after only a few years on the market.
I'm a stickler for some sort of cartoony realism in my figures, not dead on Alex Ross real, but not Manga style either (though I have a few) the block figures are the standard of a new generation of collectors, and in spirit, they are still action figures.
Even though I would never buy any, I can admire them for filling that gap between Legos and figures.