1.) You can get by perfectly well here without Cantonese
2.) A lot of doors will open if you can speak Canto
I still see many things said on these boards as facts which I know not to be true because I mix with locals.
When I lived in Sweden I learnt some Swedish, much easier than Canto, really changed life for me there. Suddenly we were able to make friends with Swedish people, (again, you'd be surprised how many can't speak English), it's the only way into the culture.
So without arguing...it's true that you don't need Cantonese to live, prosper in and enjoy Hong Kong. It's also true that you will only scratch the surface of local culture and knowledge if you don't learn Cantonese.