In general, I think taking your stud's back up in a losing strategy. It could work out, as Tennessee_ATO points out, with Spiller and Bush both playing well.
However, the reason I really don't like it is that you need so much to go right for you to win this contest. You need your studs to be studly and your flyers to fly. When you start taking your back ups you're limiting your upside, which is what is needed to win in the final 3 weeks. For Tate or Bush or Spiller to be really useful for your team, that means Foster, Forte and Jackson are most likey hurt. Which means you have a good chunk of your salary space sitting on the side line. A decided disadvantage against the other top 249 teams.
A much better strategy, imo, is to take other studs (meaning the ones not on your team) backups. That way you're maximizing your upside. Consider this example:
4 Teams
Team 1 has Foster and M. Bush
Team 2 has Foster and Tate
TEam 3 has Forte and M. Bush
Team 4 has Forte and Tate
For the final 3 weeks lets assume the following possibilities
A) Foster is hurt, Forte is not.
B) Forte is hurt, Foster is not.
C) Niether are hurt.
D) Both are hurt.
In scenario A, Team 4 has the biggest advantage, followed by Team 3 (only because he has 2 players not hurt), then team 2, and finally team 1.
In Scenario B, Team 1 has the biggest advantage, followed by team 2, then team 3 and finally team 4.
In Scenario C and D, there is no advantage gained by rostering one backup over the other.
So if you're goal is winning, then Teams 1 and 3 are your best shot. If your goal is to finish in the money, then teams 2 and 3 are your best shot.
Obviously this is a simplified example, but the conclusion is the same. Why waste money on a player who will only be worthwhile when your team has a decreased probability of winning?
Like I said, I wrestled with the concept somewhat this year. Changing my rationalization around to fit your matrix, this is basically what I determined.Using your oversimplified matrix (in which I think there is a typo, I think you mean Teams 1 and 4 are the "best", not teams 1 and 3), you are basically arguing all-or-nothing for each RB. The problem is you are ignoring the reality that any reasonable team would have other RB options available besides the 2 in your matrix. Use my team as an example -- R. Bush, Thomas, Forte, M. Bush, S. Jackson, Benson. Forte gets hurt and you still have 4ish (Benson is a little iffy, IMO) starting RBs. Replacing M. Bush with Tate takes you down to 3ish starting RBs. Staying with Tate instead of M. Bush, if Forte is healthy and Foster is out, I now have 5ish starting RBs.
So the real question, in my mind anyway, becomes:
What is the marginal value of a back-up Tate minus the #2 RB out of a pool of 4ish? [i have Tate, Forte hurt, Foster healthy]
PLUS
What is the marginal value of a starting Tate minus the #2 RB out of a pool of 5ish? [i have Tate, Forte healthy, Foster hurt]
vs.
What is the marginal value of a starting M. Bush minus the #2 RB out of a pool of 3ish? [i have M. Bush, Forte hurt, Foster irrelevant]
PLUS
What is the marginal value of a back-up M. Bush minus the #2 RB out of a pool of 5ish? [i have M. Bush, Forte healthy, Foster irrelevant]
I concluded that it is better to have as many starting RB options each week because the marginal value of a replacement RB decreases when you increase the overall pool. In my mind, adding that 4th starting RB in the form of Bush was worth more than adding that 5th starting RB in the form of Tate. I gave a marginal value of each as a back-up, at least when judged against a pool of 4-5 starters (of course M. Bush scored for me in week 1 and Tate would have scored for me in week 2).
The above obviously excludes the entire flex position because that starts to get into the value of WRs and TEs for that next marginal step.
Also, while we can debate the value of uniqueness as a general concept, I think that uniqueness has real value in 1 situation -- injured players. If the Forte herd is culled by a significant percentage while he is out and if (big "if" of course) I manage to survive while Forte is out, his performance upon return will give me a greater marginal return than it would otherwise.