We use ResellerClub
as our most used domain name registrar, to provide domain names for our clients. They do NOT require a down payment or entry fee and can be used per domain as you mentioned (as far as I know).
However, we do also use Enom as a backup, as well, for cases where ResellerClub cannot be used or it just wouldn't be practical... For example: sometimes we have clients trying to transfer their domain names away over from another resellers of ResellerClub (aka other hosts), and while pushing/moving a domain name from one reseller account to another via the RC interface is a pretty simple and straightforward process, you're not always going to be able to communicate with other resellers (if they're even still around)..
Therefore in such cases, we simply use our other registrar, which is Enom, and transfer these domain names without any problems. This has saved us countless hours and tons of back-and-forth emails with other hosts/resellers trying to help our clients transfer their domains to us. Saved time means saved money.