I do not see the issue. Carr knows the offense inside and out, can step in and possibly win a game or two if Eli gets banged up, while working with Nassib this season. I would not be shocked to see Carr ending up as a coach when he decides that playing is not part of his plan.
Drafting Nassib I think guaranteed us 3 QB's this year and maybe even next. Watching them at training camp you can see no one there is in Eli's league he is pretty amazing to watch. Hopefully he is behind center for years to come too, but you never know what happens in the NFL to a QB. I think investing in the next three years at QB is worth a roster spot