actually the IMPACT Block Rating is very important especially if you combine it with the other proper ratings....give your FB 90+ IMPACT block and he'll make the correct read a lot on which defenders to block when lead blocking.
I can show you several videos where the IMPACT Block rating effects downfield blocking or blocking out in space like on special teams....
I think some guys determined ratings don't do much because they stick to default rosters most of the year....i think the only way you can really make that assement regarding the rating is to edit and experiment with the ratings yourself....
But from what i've seen if you give a WR IMPACT block Rating of 0 and another WR IMPACT Block rating of 60 you'll see a huge difference between the two on runs to the outside when you need someone to make key blocks down field to sprang your RB
Take an ENTIRE SPECIAL TEAMS UNIT and give them 90+ IMPACT BLOCK and watch your return game get better.....right now the game's default ratings for most players beyond the OL is like 40 for IMPACT Blocking and that is why you hardly see a guy like Hines Ward be replicated in NCAA Football