There is no excuse for it other than to differentiate the cheaper MacBook from the other models, in an attempt to force users up to the next price point.
Which is exactly what I said (typed!) - if the Combo drive didn't exist Apple wouldn't be able to charge extra for the Superdrive models.
(It is the same reason that Starbucks offers soy milk and extra shots in their coffee at an extra cost - some customers will pay extra for the privilege.)