By giving people who do not earn enough in your eyes tips you support the fact that they don't earn enough because it gets abused as an excuse not to pay them fairly.
Who does and does not get tips is also very arbitrary. Many low paying services you profit directly from go without tips just because the person doing them does not ask you to pay for the service. A cleaner or a dishwasher, for instance, will not walk up to you in a restaurant, so they don't get any tips while they receive about as much payment as a waiter.

The only time I tip is when I know that all workers will receive equal shares from all tips and I feel particularly sorry for the hard work they do with little payment. The last time I did it was in Egypt where there is no such thing as fair work conditions.