Unless you want to go over 7spd or your a heavier rider I see no reason to switch. FW type hubs have served very well for years and the system works OK.

To us C&V riders who use 7spd setups the the only real advantage to a cassette is simply that the hub bearings are closer to the end of the axle making a stronger hub and making a bent or broken axle les likely. Shifting may be inproved slightly due to the ranps on a shimano cassette, but if your current setup is using Synchro with a Regina or Suntour FW you'll have to start hunting the correct insert to go with your new hub.