Hi Steven,
I don't think I've read a trip report yet about the WCT that indicates that a GPS is necessary. I don't think you even need to bring your own map as they provide you with one during the orientation session before you are let loose on the trail. I like to have the GPS with me so that I can keep track of travel times and speed as well as distance to the next waypoint and ETA.
I also do geocaching but as the WCT is in a National Park it doesn't look like there are any geocaches on it (geocaches aren't permitted in Canadian National Parks for some reason).
That model of GPS (Garmin GPS 60CSx) also has tide predications on it for most coastal areas. I've also load the TOPO maps for all of BC and Alberta onto which also has some trails and logging roads marked although they need to be updated.
Mainly though, I'm a bit of a gadget geek and I like the (false) sense of security that knowing where I am down to the meter gives me.
On the otherhand, it does eat the batteries pretty fast and I don't think there is much chance of getting lost on the WCT given the number of people that do it each year so I may leave it at home and save the weight.