I'm still new to the software, but I believe that the expansion packs for SD just add sound libraries and additional presets. It's the base SD software that has the engine to run it all, and that's what you're paying for. It just happens to come with a complementary library of raw hits.
So if they were to make the SDX's stand-alone, they would have to package the core software with them. In other words, you'll have to buy the core software regardless.
That's what I think the situation is, but I don't know. It'd be neat if it was done like the EZKeys line where you get the software with whichever expansion you initially get, unless the core SD library is somehow bound to the functionality of SD itself.
This is why I stick to the EZ line. It's easy. XD