Have you ever taken a food order for 8 people, with 3 courses, different drinks and then delivered the correct order to the correct people after you've carted it back from the kitchen
Well in that case, I've never had good service and I shouldn't feel the need to tip, because they take the order and when they start bringing things back, its 'whose was the salmon' to which people start putting their hands up.
I rarely go back to the same restaurant because there are hundreds of restaurants in my city and I like to try different places, so my tipping behaviour is never going to influence the sort of service I will get.
I do tip most of the time, because I feel obliged to, especially when eating in groups, but in most cases I can't see what waiting staff are doing to deserve extra, over other similar jobs. Fetching and carrying food and being pleasant and friendly is the basic requirement of the job.
DP used to work in a fairly high end bike shop. He could spend up to an hour with a customer discussing their requirements, measuring the customer, helping them test ride, advising on accessories, building the bike, which may have cost several thousand pounds, all for a few pence an hour above NMW. Once or twice a year, a grateful customer would bring in something like beer, chocolates or biscuits to say thank you, but it was extremely rare.
I have elderley relatives who have carers. On more than one occasion they have tried to slip them a fiver as a thank you, but it has been refused, usually because, other than token wine or chocs type presents at Christmas, they are not allowed to accept tips.
But yet people insist that it is necessary to give a few quid extra per person to waiting staff for doing basic tasks that they are paid to do anyway 
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