Definitely more useful.
Currently I use the expresscard mostly with an expresscard ssd to hold my iTunes library and iChat logs. Because I wipe my hard drive a lot and because my iTunes library is massive, I normally would leave the library on an external hard drive which was annoying to have to plug it in all the time to use it. At least the SSD is pretty much flush with the notebook so I leave it in there.
I also use it with an eSata card but that's not as often.
Course there are tons of other stuff you could use it for like even buying a flush with the case 21-in-1 card reader instead of being limited to just SD. You can use it with external video cards (though buggy at the moment with some macs), extra firewire or extra USB ports, external sound card (Creative's is only for Windows, I believe, tho), heck there's even a PCMCIA adapter.
All in all, even if I only were to use it for SD card reading, I'd still want the expresscard slot as a "just in case". There's just so many choices.