Hi Brit Expat - no they don't. there was many a Hong Kong chinese that felt the cold wind after 1997 when they were sneered on and excluded from jobs in their native SAR Hong Kong (special administrative region) by the big wigs from over the border - because they only spoke cantonese.

by the way, cantonese is not the only other widely spoken language in China, there are many many many places with dialects where that is the only language understood and spoken and someone speaking mandarin might as well be speaking marshan