Space Marines are more than Special Forces. The Special Forces, (as we know them today), are the Stormtroopers or the Cadian Kasrkin, (read the "redemption Corps" novel). They are Crack Troopers. More than mere men.
Their role in the battlefield would be similar. Yes. But they are not just SF.
I think that when we think in the DW we must think in Black Ops operatives. Usually they will act in secrettly ways. Not been seen. No proofs. Only when the situation turn really desesperate is when they will be deploy openly in the battlefield, (like in "Warriors of Ultramar"). They seek their own agenda, (or the one of the inquisitor of the Ordo Xenos).
Anyway, in the last Game designer Diary the following is said:
"Some Battle-Brothers may gain enough renown to be granted the coveted honour of the Crux Terminatus, a special distinction that includes training in the use of powerful Terminator Armour, whilst other Deathwatch Space Marines might, at rare times, choose to use lighter Scout Armour when stealth is paramount."
So, it is possible the concept of the Deathwatch Scout.
For anyone that think that it is not possible for a SM in Power Armour to be sneaky, I suggest them to read the DW short story "Headhunter" from "Heroes of the Space Marines".