I left RIM in 2012 so I could not confirm that this is how the browser works in new devices released today. But given the success and advantages that having the html browser gave us there would have to be some really compelling reason (or $$$) to abandon it. The group I was part of was formally TorchMobile and as a group had significant browser experience having written a fair number so we understood the hassles of maintaining two projects. Being able have webkit developers help with part of the browser made development go faster on both sides (webkit devs could easily abuse the browser to test new feature). Even if it isn't used in the browser as mentioned at the bbjam the weblauncher application was used by not only the browser, but for a number of other purposes.

I can only imagine there is a fair amount of overlap in Mozilla where there are two developers working on the same thing, one for xul, one for html.