If I'm seattle, I probably have 4 backs on my roster. 2 are guys that I should feel comfortable giving the ball to 20 times a game. The third would be the pass catching/change of pace back, and the fourth would be a fullback that is a threat with the ball in his hands as well. Why a FB? Their best running seasons with Lynch were when they had a FB. Same has been true for Arkansas as long as I've been a live. However, not every back does run best with a FB. Some perform better without.
Now, if Lacy is one of my top 2 backs, then I want to have a guy stashed away on my practice squad who I trust because I'd be a fool to not have a backup plan when Lacy gets injured. Carroll wants Lacy to be a big back and in the 240's because he thinks that is where Lacy is best. Under my scenario, that could lead to a scenario where Lacy is the big back on the team though they don't use a FB much. With Rawls as the primary back, Procise locked into the receiving back, and Lacy as the power back, you wouldn't necessarily need a 4th back on the roster if you aren't using a FB. However, if they did, Alex would likely fit into that last spot.