You can add expansion cards for more ports like firewire 400, 800, USB 2, or esata. Also if a new input were to come out after your machine was made you can add it in using that slot.
As for the ssd, 32gb seems to be the popular "larger size" right now for it.
I use mine for eSATA since my 1TB Samsung drive would max out Firewire 800. Another reason is that eSATA enclosures are a lot cheaper than Firewire 800 enclosures (like $30 instead of $70).
You could also use it for a memory card reader. I think they make some that sit flush with the outside of the computer. They all use USB, though, so don't expect them to be any faster than a USB card reader. (expresscards have a PCIe 1x lane and a USB connection, and cards can use one or the other or both.)
I just came from circuitcity. There they have expresscard adaptor for memory stick duo. Is card flush when inserted? Does anyone have it here? Would buy it, but there are 2 things to consider- no return and I am not into sony crap especially memorystick. But if it fit flush I probably would buy it.
edit quick image googling turned NO! Card is not flush...