For example, I would love to interview PR professionals. Compared to parents, I would venture a guess that they're much harder to find on MT. I wonder if Facebook ads would work better.
There are three big issues with using MTurk for business research. You can't do much to refine who you get as respondents, the respondents are incentivized to complete the task without regard for the effect on your data, and you have restrictions on what you can do due to the anonymity rules.
If you're following the way of the niche, MTurk may be too problematic compared to just slogging through some cold calls. The benefit for more broadly targeted projects is that it lets you grab a large sample size with fairly low effort.
(Speaking of which, if you are an entrepreneur or an early employee at a startup (like <= employee 3), I'd love your feedback via a quick survey! http://www.surveymonkey.com/s/F7W9P5P )