Is there any reason to get the +2 to grenade count for rank 5 of a grenade for any squad member?
Yes, Squad mates have limited grenades, which is why they don't use grenades on their own. Note that if you don't manually use your squadmate's grenades, there's still no reason to upgrade, as they're not using them automatically.
Squadmates gain grenades when Shepard does, so it makes sense to keep squadmate's grenade count at Shep's level. Assuming you use the squad's grenades.