What's the selection of Redbox DVD's at grocery stores like? I have a hard time believing it's anywhere close to Netflix's extensive DVD selection.
Also, Netflix has various plans: you could rent one DVD at a time, two DVDs at a time, three DVDs at a time (and maybe more). If you have three DVDs out at a time, and watch one DVD per day and mail it DVDs back as you watch them, you won't have to wait long before your next DVD arrives in the mail, possibly while you're still watching the rest.
In my own experience, watching a DVD per day is pretty ambitious. Odds are you'll be watching way less than that, and there'll be multiple days between watching DVDs, in which case waiting for your next DVD to arrive in the mail won't realy make any difference.