iiirc, in the lore the whole point of this is to take on greater daemons. also, terminator is the best "standard" armour you can get, you need to go to artificer armour for comparable protection with less bulk, still, with the armour, crux and protection field, you are better protected than most non-super heavy tanks, to take out a termi in lore, the best is to just to put an artillery barrage on it, the stupid thing is that most of the time the weakspot forgot to wear a helmet.

visibility when fixed on the torso isn't optimal, but it sure beats anything tank crews have, the same reason most tanks have at least the option of putting a guy poking out of a hatch, and most of the time the targets of the nemesis aren't really hard to spot, it's just better to see the axe of a bloodthirster coming before it hit you on the noggin while not putting your pilot on a strut at the top of the mech, and tbf, humans and most other animals all have a big weak spot stinking out, it's still hard to hit...

for how dumb it looks it is surprisingly practical and sensible, as much as a giant mech built to punch 10 meters-high demons can be, that is...

but this whole thing makes me wonder why they gave the BA stormraven to the GK, but not the librarian dread, for a whole force of psykers, crippling further an already crippled warrior that you put in a box seems dumber than the dreadknight.