I've been pretty impressed with the service, and so far it's been worth the price in terms of ROI. I only publish high-quality content on my sites because obviously I also want to give visitors a reason to subscribe and link in, in addition to getting love from google.
With regards to paying freelance writers, look at it this way: NY Times pays a lot of writers across their sites to produce content--writers who are no more invested in NYTimes than the salary or payment they receive for their work. I could never put out enough content on my own to stay competitive. It's simply a division of labor.
I agree with your point about the risk in losing a clear and consistent author voice when using multiple writers. For that reason alone I might avoid it, coupled with Thomas' point about volume of content being low-value when it comes to author rank.