I don't use deadlifts for my lower back at all.
Do you mean that you do stiff leg deadlifts, or do you mean that you don't use regular deadlifts as a back exercise?
Sure, they both work a ton of muscles, and they use a lot of the same muscles- But to differing degrees. When you do squats, does your lower back feel as challenged as it does when you do deadlifts? If you're using proper form, the answer should be no. Deadlifts target the back more than they target the legs, and squats target the legs more than the back. I wouldn't cut either one of them out from a weekly routine, or switch them up from week to week- Even though it seems like they might get more rest that way. Think about it- Plenty of muscles are used in other exercises on days when they're not being targeted- For example, as Jenn pointed out in another post, I love to talk about the relationship between shoulders, triceps and chest workouts. All three are used in chest workouts, but you still have to hit the other two during the week- And even then, shoulders and triceps are both used in shoulder workouts also. The thing is, in my opinion, all these muscles that are used on days when they're not targeted aren't really challenged on those days- They're just used as stabilizer or support muscles, it's not like they're getting a full workout. Again, after doing deadlifts, do your legs feel the same as though you'd just done three nasty sets of squats? Same muscles are getting used, but at varying degrees.
Just my opinion.
