I wouldn't mind having a big scary thing standing behind me when I talk to people. My Inqusitor favours brutal shock tactics. Plus when I'm off my head on drugs we can sit and dribble together.
Regarding the use of Space Marines, I just read this: http://www.games-workshop.com/MEDIA_CustomProductCatalog/m1320038_Using_Space_Marines.pdf . It makes you realise how much you lose when you play a Marine. They can't sneak around, they can't make deals, they never retreat and they don't understand most human emotions. Pretty boring for a roleplay character unless you're in the right situation.
Being a longtime player of and fan of marines, Asmodeus, I'd have to respectfully disagree- mostly. They entirely can sneak around. Look up Kayvaan Shrike
, who not only is capable of hidden, guerilla type tactics, he prefers them. It really depends on the Chapter the Marine comes from. Some Chapters absolutely refuse to use stealth, viewing that as showing fear, hiding from the enemy. Others, like Shrike and his Raven Guard, thrive on stealth.
You're right that they can't make deals. They're the retribution of the Emperor given flesh and form, and they do not make deals with enemies of the Imperium, but that's part of why the Inquisitors love them- if the Inquisitor says "You're going to wait to destroy that heretic until we uncover the greater plot behind their madness", the Marine bows to his authority and waits, as the Inquisitor is secure in the knowledge that the Marine won't betray his authority or the Imperium's cause.
retreat. However, they do not generally feel fear. If a retreat is tactically advantageous, they do so orderly and without panic. It takes something on the order of a God for them to feel fear- and even then they'd usually grit their teeth and charge, hoping to die on their feet with a battlecry to the Emperor on their lips. In the new Codex for the Marine army list, they have a special rule called "Combat Tactics" which allows the Marine player to choose to fail Leadership & Morale checks- as the Marines on the table decide getting into cover would be an advantageous decision under heavy fire or other similar reason to fall back, its just never actual fear.
And they do understand human emotions. This one would be a bit harder to explain and display, but I highly recommend reading "The Ultramarines Omnibus" by Graham McNeill. I'll use an example from the first book in the omnibus... The Captain of the Ultramarines 4th Company, Uriel Ventris, investigates a planet attacked by Eldar raiders with an Inquisitor in tow. He finds one survivor, barely hanging onto life, delirious and speaking almost incoherently, but seemingly prophetically. The Inquisitor (who isn't present) would've wished Ventris to keep that man alive and bring him with them so he could be further questioned by the Inquisitor (you find this out later), but Ventris orders the man to be given the Emperor's Peace, at the man's request, and orders his Marines to find his wife's grave, so he can be buried with his woman.
And if that doesn't show a human capacity for feelings, I'm not sure what other example would
As for Max's suggestion, I'm not sure who both Inquisitors are... Asmodeus and...? I'd be totally happy to go with a heavy weapons type marine- thats honestly how I play my table top army, overloaded with heavy weapons- and since Scouts can use Heavy Bolters/Missile Launchers in the army list, I'll do that so there isn't a Sororitas, a Grey Knight, AND a Marine all wearing power armor.
Scouts would also have an easier time with noncombat necessities... stealth and recon is part of their training as a Scout, so donning regular clothes and walking amongst the populace is more right up their alley