I've read that focus groups are a poor way of conducting user research as it attracts professional focus groupers who work the focus group "circuit" for a living. This obviously has major drawbacks as you're getting "expert" opinions instead of a random sampling of typical users. What's more, if a person participates in focus groups regularly or does it for a living, then their feedback is likely to be influenced by past sessions.
But I don't see this problem as being any different for user testing. I often see ads offering money for participation in beta testing or to be a user tester. Sites like usertesting.com offer cash rewards to users who sign up to test sites ($10 per site), so presumably their test subjects are also going to be "professional" user testers who are participating in a large number of studies to earn more money. Other companies like UserZoom source the recruitment of users to "professional panel companies", whom I assume also compensate their panel members.
So is paying user testers a good practice, or does it compromise the data that you gain from it? If it's not a good practice, then what alternatives are there to get large numbers of unbiased test subjects? Would prize raffles be more effective?
I know a lot of sites simply randomly select visitors to participate in user tests. But what if you're hired to conduct usability testing for a new service that doesn't have enough traffic to recruit enough beta testers?