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!

You have to think beyond the OS. How many times are you booting up your computer (or putting it to sleep) a day?

Now how about launching and working with files? A lot of times a day I guess.

You don't want to keep ONLY your OS in the SSD, you want to keep often used apps, your music if it fits (and you access it very often) or your photos, etc. That's what takes time on a hard drive. On the other hand, since they are still not big enough to replace mechanical drives (and it they were the prices would be prohibitive) then you still have to have a traditional drive for long term storage.

Apps in SSD Drives launch in 1-2 seconds. Everything is faster.