And so you are willing to bet an early FF draft pick(based on top 10/15 RB finish) on Turner even though he has been a career back up who only plays when SD is way ahead and the opposing DEF has all but mailed in the game?
I'm not sure who you are thinking of, but Turner gets a good chunk of his carries in the second and third quarters. Half of his career fourth quarter carries have been when his team was within seven points one way or the other. Also, to call him a "career backup" is to mischaracterize him. His "career" has only been three seasons long, and the reason he has been a backup is because he happens to be on the same team as the best back in the league.
I'll take him as a #3 RB but at top 10/15 you are talking a #1/#2 RB in terms your FF draft. I think there are better risks for RB1/RB2 before I would pick Turner early. JMO.
I don't think GB is really taking his fantasy value into much consideration.