Your statement about missions requiring different types of hardware isn't wrong, but having a generic platform for creating probes with different mission profiles isn't a bad idea. Think of it like a car platform which enables you to put on different bodies, interiors, etc. to suit the tastes and requirements of purchaser. If you had a standardized parts bin you could select whatever you needed to meet your mission objectives.

That said, usually you would do this to leverage economies of scale, and I think there probably isn't the budget or demand for enough space probes to make it viable. Who knows though, if the price of space launches continues to plummet, maybe it starts to make a lot of sense.