We always tip. My parents always taught me that it's a small polite way of thanking someone.
Just pondering out loud, but could it sometimes be the other way around? Rich diners who see no need to actually show thankfulness in their words or actions and treat the waitress brusquely and with a frown - because they figure that chucking an extra £20 on top has bought her servitude and submission to them as their inferior?
Of course, you would hope to have both, but I wonder if the average waitress (or anybody else in a job where people often tip) would prefer a rude, terse, superior-acting customer who throws them a generous tip at the end or a friendly, respectful family who engage as equals throughout and express their genuine gratitude verbally at the end, before settling the bill to the exact amount that they've been charged and no more or no less?
I would hate the North American system. From what I've heard (if it's true), in hotels, it takes you a lot longer to get on with what you're there to do, because you have to keep waiting for people to do very simple things that you can probably easily do yourself - parking your car, carrying your luggage in, opening doors, pressing buttons in lifts - and then having to get your purse/wallet out all the time to give them money as a reward for delaying you. Some of them are non-jobs that you didn't need somebody to do in the first place, much less paying them extra on top.
The expectations in restaurants and bad-mouthing/complaining if you don't give what's 'socially compulsory' just sounds horrible. If the tip is already a 'set' amount, why not add it on to the bill? In fact, why not go the other way and not bother having a bill at all: just make everything nominally free but with the high-pressure customary near-obligation that you need to pay them the amount they're expecting, but for some reason not keen to present to you as a bill?