MTurk is a godsend for ML research and is a huge game-changer. For every other project where the problem is "that sounds cool but we don't have enough labeled data" the answer nowadays is "just turk it". Sentiment labeling, qualitative comparison, error identification, and tons of other traditionally data-scarce tasks are made trivially easy (at the cost of some money) with MTurk, and it's pretty much a win-win for everyone involved too!

Now the ethics as far as exploitation are definitely important, but I think the design of the site handles things quite well and makes everything fair for all parties. If you feel a task is underpaid, there are enough alternatives that you can just not do it. It's also true that there are many international turkers for whom $8/hour or less is still solid pay. Then there are also many third-party tools that allow turkers to see which HIT (task) requesters have good track records (low rejection ratio, good pay, etc.), and the site's own tools allow requesters to avoid turkers with bad track records. In my experience just browsing through tasks, heavily underpaid tasks don't tend to get done (for example, writing a 100-word summary for $0.50).