I think people are missing two very important points about blasts and strategies.
It's not about what a 2" marker does when someone uses unit coherency to full extent. It's about creating situation where this is not possible or leads to some disadvantage for them.
A good example is troops disembarking out of a transport or - even better - deep striking. If they appear and shoot they will be not be spread as well as after moving normally (speaking of an average 10 model unit). So by having blast markers you get the advantage of either getting more out of your blasts or forcing him to run to get his models spread out. But then he can't fire.
The other point is that area terrain is gone from 7th (with small exceptions like craters). Many haven't even realized this yet. This means that your models have to hug more to use cover than before. And this means more hits with blast markers. Or the opponent spreads out, but will lose cover for some models.
It's not always about just calculating the odds for the single rule, it's also about thinking about which situation to create, which decisions to force upon the opponent and THEN calculate the odds