There will come a time SSD is standard but i guess Apple will want to drag out the price hike for users to pay for the full 768GB SSD by keeping the fusion portion of the SSD low (128GB)
They could increase the SSD as prices drop in the future as most of us have lots of storage (games, video, music, etc) I don't understand people saying they will buy a big iMac but keep below 120GB on their system so it uses SSD as i couldn't survive with that much space.