I just counted the number of entries on the list of Mozilla-based applications page and came up with 164 different applications (I went quickly so I might be a bit off). I think this number is an interesting data point in the discussions about why people choose to use WebKit instead of Gecko.
Another interesting thing to keep in mind is that there is more to Mozilla’s technologies than Gecko and XUL. Google may have chosen to use WebKit instead of Gecko for their browser, but they also decided to use NSS and NSPR.
I think there is certainly a lot of room for improvement in helping people reuse the interesting bits of code that the community is creating, but even with some hurdles for developers there are lots of cool things going on.