When I got Farley who was the 1st one I could register, I did register him with AKC. When I got Chloe I didn't bother submitting hers for the same reason as you. Then I found out it's fun doing obedience competitions with them. In order to let Chloe compete I had to apply to AKC for an ILP# since I had waited too long for the conventional registration.
So I'd say if you are 100% sure you won't want to ever enter your furpal in any AKC sanctioned events (obedience, agility, etc.) it really isn't necessary. But if there's a chance you might want to, go ahead and register him and the option to participate is open to you.
Actually, even with rescues, as long as they are purebreeds, you can get an ILP# for them from AKC as well.