My iTunes library is 100 GB. Between iPhone apps, digital copies of movies etc. That grows a little every month.
My mother can get by on 40 GB. I know that there are people that can get by on small amounts. The fact remains, storage requirements only grow... They almost never shrink. SSDs will be great with you at least the current amount of storage that people use for a reasonable price.
I'd bet that if you took a poll of MBP 15 and 17 owners, few would even consider trying to get by on 120 GB.
EDIT:
I should mention that most of the people that I know with MBPs and MB use them for photography, video editing and music. I know that my experience is not everyones experience. Excluding casual users, I personally don't know any MB or MBP users that can get by today on 120 GB. We are talking about systems in future. People would be purchasing this workstations for use years from now.
Also, lets be honest, do you really see ANY computer computer telling the general public "Hey, yeah we are going to give you half of the storage that you used to get for the same, or more money... Oh by the way, we have no clue how you'll migrate you 500 GB of data to the 256 GB SSD". It COULD happen but you'd have to be a marketing genius the likes of which the world has never seen.
I could see it on the lower end models... not MBP 15 and 17s across the board.
PS.. Didn't mean to sound snotty