This would actually be a good feature in a number of ways. Especially if clients were able to opt to have their bills taken out weekly instead of monthly, so that WHMCS could automatically take $xx each week, placing the account in credit, until the bill is issued and then take the remainder of $yy out of the clients credit card when the bill is due. It would be an opt-in feature chosen by clients who would like to spread their payments out on monthly products. Wouldn't be so useful for cheap hosting packages, however for more expensive dedicated servers, colocation etc. it would make life a lot easier for clients being able to select a certain amount to be deducted from their accounts each week/fortnight. You would make this a client selectable amount.

As well as making it an option to set the product price to a weekly amount as well during product creation in the admin area.

Sorry about my bad grammar and missing spacing etc. typing this on no sleep.