Manufacturers put rye grass in mixes because it will grow anywhere and hungry deer will hit the snot out of it when it first germinates. However, nutrient content is poor. I NEVER recommend any type of rye grass.
As far as deer nutrition, I agree... but it can be a valuable tool if used for the right reasons...
I wasn't kidding myself using it for feeding deer, but I chose rye grass for my new plot I pushed in last fall. It wasn't completed until Oct 1st. I wasn't able to sow it until Oct 7th. I needed something that would grow fast, prevent erosion, and produce a ton of biomass for the following summer crop. That late in the season, I didn't have confidence a cereal grain would do what I needed... so I went with ryegrass (with clover and radishes). The ryegrass choked out the clover and radishes, but gave me a ton of thatch to sow my early summer crop into. It was easily terminated with Gly, and left me with a solid inch of biomass to build topsoil. I'm thrilled with how my summer plot turned out following it.
I won't use it this fall on this plot, but it served its purpose. The deer loved it, but they also had another 10 acres of nutritious foods within 800 yards available this past winter. The biggest problem is letting it go to seed and reseed itself on the edges of plots if not terminated with herbicides