I honestly love Apple Pay, but I don't use it super often because it makes me self-conscious. I feel like a bit of a dick for some reason. If you try to do something "weird" and it doesn't work, people get annoyed, or it feels like an imposition. Even when it works, people sometimes make some kind of comment. Relatedly, it's unpredictable whether it will work or not. It usually does, as long as the card machine says…
Feeling rude happens because when you suddenly produce your phone or watch, you suddenly reveal you’ve been thinking of the transaction differently and maybe that the cashier was incorrect or ignorant to assume you would swipe your card. You are unmasking yourself, in a sense. Proactively mentioning it lets them work with you and share their knowledge about the terminal if it’s relevant.