What is the point with iMac + SSD?
The base model of 21" iMac only has one drive bay. So, a 20GB SSD wouldn't be very useful there. If you want an SSD, you have to get one big enough to hold all your data.

As for the 27" iMac and the upper model of the 21" (which have two drive bays):
Performance. Yes, you can fit the OS on a small SSD, but that's not the only thing that will benefit from being on a faster drive. Any operations that involve a lot of reading/writing will be faster if they're working with files on an SSD.
Convenience. Unless you set up your apps and Finder especially well, it will take a little more time to get to stuff on a secondary disk.
Quietness: SSDs don't chatter like HDDs. If you have both an HDD and SSD, you'll still hear the noise. Silent computers are nice, IMO.
Apple-ness: One of the things many people like about Apple computers is that things are mostly seamless ('Just Work'). Having one big, fast, quiet drive is more seamless and Apple-y than one small, fast, quiet drive and one big, slow, loud one.

One more reason Apple does it: Because they can: Add SSD. People buy SSD. Profit!