I think I will be the first to say that good game mechanics and realism do not mix.

You can have good game mechanics at the expense of realism or the other way around, rarely can you have both.

Beside, this is not a CoD or Batlefield game that try to be super serious and market it as authentic experience, this is a game with aliens, demons, space elves, space orcs and genetically enhance super human in power armor, realism should be the least of things to aim for.

The 40k universe has it's OWN realism, just because it isnt OUR realism doesnt mean we should consider it a null point

I, for one, hope they keep these mechanics. I don't think the game will feel nearly as good if my huge, ironclad Space Marine can't roll clunkily away from small-arms fire; with serious weight behind it. I hope they add two different methods of 'dodging', personally; a roll as explained above, and a dash/evade step (possibly utilized by pressing a movement key twice in rapid succession).

Either way I'll be fine. As long as they don't let the Marines use cover!

And why would/should the Marines [be forced to] just stand out in the open to be shot at like clay pigeons?

They were using cover in that livestream and it looked pretty good. Astartes can use cover if it is big enough although they dont always need to as they arent flimsy weaklings like Eldar

That was a joke; apologies if it wasn't too clear. On a slightly more serious note, however - I will not hide from bullets, I will face them head on!

I actually didn't catch that.

Thanks for clearing that up.

Remember how unrealistic people thought waist high cover was? Well we no longer have waist high cover!....to normal humans.

We can now use the much more realistic head high cover.

I'm all up for Space Marines Rolling as long as they make the animation look heavy, a good example would be Dark souls when wearing heavy Armour you roll you hit the ground at force and they get back on your feet.

I agree in general with your point (gameplay > realism), but here this is a no problem, because the OP even proposed a backstep which has the same gameplay mechanics.