Were do you work? If you're in the industry then my apologies but my experience differs greatly from yours, at the very least (and you must work for a cool studio) If you don't work in the games industry however, you're being very naive.
Concept art for game purposes its not something meant for marketing and as such, consumers never ever see it (for the most part, there's exceptions, look below). We basically are told to paintover whatever fits the image were looking for, and if you're doing a photorealistic game that means paint over photos and collage photo reference together on a daily basis. Even with other games too, Ive had many assignments were my instructions were to take screenshots from Team Fortress and paint our stuff over the image in order to get to the look we wanted.
In a game studio its not about credibility as much as it is about getting to the look the AD wants the game to have, it absolutely does NOT matter how you get to that point, and photo paintovers are the norm, sorry to disappoint you.
As a disclaimer note that I'm talking about photorealistic games, which are the majority of what the industry produces. Yeah my 90% comment was an exaggeration and places like Blizzard probably don't dabble but as an example, here, some Force Unleashed concept art:
See the face and the arm? THAT'S FROM A VIDEO CAPTURE FROM THE MOVIE. Take the capture, clean the background, put a green hue over it, paint some details on the face and do a quick rendition of the body and you're done. Id be shocked if that took much longer than an a couple of hours and you know what? Thats perfectly fine and acceptable, hell, its smart and it was done by a mayor studio for a mayor game. If that doesn't prove my point I don't know what does.
