Why is "clicking" frowned upon?

I think most would probably agree that regardless of their preferred style of control there's a quicker response time in general when the action only requires a key to be depressed, rather than having to move a mouse. It's probably up to some debate, but the issue shouldn't be how you choose to use controls in the game, it should if you're comfortable with it and if you feel you're able to perform adequately for how you choose to play. If not, think about finding ways to improve.

As far as it being frowned upon, it's just another way for some jerk to judge you (on the internet, in an RPG, on your control preference no less) by saying you're doing things inefficiently, wrong, slowly, worse than they can, etc. They're bullies, and while they look like they're everywhere because they're loud and like to make themselves known, they don't account for everyone, and they're few enough to ignore easily.

You can waste your time attempting to argue with them, or ask them what they're even talking about, but it's just that, a waste of time. Play the game how you want, and how you enjoy playing it. Everyone else that's going to tell you you're doing it wrong isn't worth the energy it takes to blink while reading their text.

Are you enjoying the game how you choose to play it? That's all that's important.