This is something I’ve heard more than once – usually by people who don’t speak Cantonese (or Mandarin for that matter). The short answer is “Of course you can, silly fucking rabbit! What do you think ’em funky-looking sinograms like 嘅, 唔, 冇, 嘑, 哋, 叻, 攰, are for?” They sure aren’t for bathroom decoration.
A more elaborate answer would include not just odd sinograms used only in the Cantonese language, but also words that don’t make a lick of sense in Mandarin: 而家, 鍾意, 脷, etc…
There is a form of Cantonese called “Written Cantonese” — basically Standard Written Chinese with a hint of Canto flavour, and pronounced in Cantonese when read. Ugly and not really the Cantonese people speak every day and relate to. The announcements (verbal and written) in the MTR are a good example.
But along this stilted language there’s a vernacular written language that mirrors spoken Cantonese. It’s used in ads, SMS texts, IM chats, casual emails and similar non-formal communications.
“Real” Cantonese in ads may have been around forever and a day, but I have noticed an increase lately. Whether this increase is real or perceived, I don’t have enough background to be sure. This ad I took a picture of in the MTR is a prime example. “我捐嘅錢、下一站會去邊?” (The money I donated, where does it go next?) Run this through Google Translate for a good laugh… The use of 嘅 and 邊 (short for 邊度) makes this sentence a vernacular Cantonese sentence. Written indeed, and plastered all over HK.