The last laptop I had (A god-awful NEC Versa Note) had a built in network card, so I used my cardslot for a modem.
The way I see it, Laptops are getting more and more fitted out with tech as standard that previously was only an option, so cardslots are getting more obsolete.
As for speed, the first zip drive I got had the option of PCMCIA card connector or USB. The card was woefully, woefully slow compared to the USB connection. Not sure if thats a good example.
As for what cardslots can be used for now, I like the idea of having a MS reader, but I think you can get GPS cards which would be kinda cool to have, especially if you do a lot of driving, just sit the laptop on the seat beside you and run it as an in-car GPS that keeps sending you down dead-end gravel roads.