I have in the past, but not often and I stopped entirely last year. When I need a taxi I accept the costs of getting a black cab. I don't buy from Amazon either. I try and select goods and services that aren't based on gross forms of exploitation, but it is hard to avoid in our society.
If you actually caught Ubers and talked to the drivers I don't think you'd compare the system to zero hours contracts (and I don't think they'd agree that it is a gross form of exploitation either).
Would I be right in thinking that you've never/don't use Uber?