I think that it is important to highlight that the role of being a tester, is different to the job of being a tester.
There are many situations where you need testing, but you simply can't afford or are unable to have dedicated testers, i.e people who have that job 100% of the time.
- You simply don't have enough testing, or resources to be able to justify the cost of a dedicated tester (e.g. 2 man startup)
- You are in an organisation where you actively practice role combining, i.e. your UX or PM might act in the testing role.
- You use Business Analysts to perform the testing
All very valid reasons not to have dedicated testers. Yes, dedicated testers are the preferred option, however that doesn't mean that they are always appropriate.
If you follow the mantra "Quality is everyone's responsibility", then you can actually get great results without dedicated testers under certain circumstances.
Personally I prefer NOT to have a dedicated team. I think that the feature team approach, where testers are one role embedded alongside developers is a much better way of doing things. A separate QA team can build an us vs them mentality, which is not healthy.