If you run more than one http server for 1 site, each server has its own xcache, so if you make a change in the admincp and save it, depending what server your on, its only going to update the cache on that server and not any of the others, there are if i recall right, 13 or so different var's that will be stored in xcache when using that hack, just like it would be for running vbulletin with file cache or memcache when running a site with multiple http servers.