Why?
It's not like the two second year players havent played the game. They have played it since they were in HS, College and last year in the NFL. I have never thought much of that "they need an experienced veteran." They have coaches, and thats what coaches get paid to do.
Give me one good logical reason why they need to have a veteran.
I think you have a point that it's not necessarily 'needed,' but it seems to be a Belichick M.O. if I'm not mistaken.
When using a RBBC and having 2 young guys (possibly 3) who have extremely limited experience in the system and NFL in general, it never hurts to have someone who can come in and get the job done if and when called upon. That may be due to injury or just ineffectiveness.
I think another thing a vet provides besides experience and starter capability is cost effectiveness. There are many vet RB's who can come in and offer what we need here (500 yrd max) in limited situational carries, who have played in big game settings before and have some tricks of the trade to offer, while being relatively cheap too.
It also helps with the leadership aspect in the lockerroom.
I don't know if this has bearing or not, but it's also possible that Belichick has learned from past mistakes when having too many young players at the same position at once. Of course that's just pure speculation though, and nothing more than a guess.
I think that you have a point, but I think that BB probably feels that bringing in a vet has its benefits.
If I had to pick from all 3, it would be:
1.Grant
2.Hightower
3.Addai