Here are two things I've said about Action Mode on a different thread:
Firstly, just because it's in the game, and it's the top option so "OMG BIOWARE WANTS US TO PLAY THE DUMBED DOWN VERSION BECAUSE IT'S THE TOP OPTION!!!" No. Stop. You should already have learned not to just press A on the top option from playing the first few Mass Effects. You want to play the 'proper' game? Go ahead and play the 'proper' game. Stop complaining that people are going to be buying it who aren't necessarily RPG enthusiasts. And on that note...
...it's entirely possible, though unlikely, that someone might be playing the 'Action' mode and thinks, "Hmm, I'd never say anything like that... Didn't the game say something about 'automated' dialogue? Let's try RPG mode and--Whoa, I'm really digging this!" Unlikely, sure, since the stereotype of Call of Duty players is that they're hyperactive, foul-mouthed eight-year-olds, but it could happen, and that can only be good. Besides, don't you want a game you really like to sell well, or are you not allowed to like popular things?