Why pay for a professional to do the work when you can get it so much cheaper?

Collaboration

For one thing, design is a collaboration.  With the design contests out there, there is little to no collaboration between the client and the designer.  Basically, a request gets put out and hundreds of designers compete for the one picked.  Needless to say, this is unfair to the designer, but it is also unfair to the client.  Good design is the result of talking, listening to, and collaborating.  Without feedback going both ways, it is not likely you will be getting what you want.

File Types

Many of these logo sources will not supply you with the final files you will need for every aspect of your marketing.  Many times for such a low fee, all you will get is a low-resolution file that can only be used on the web.  What about when you need to get brochures and business cards made, and how about large format signs?  This is a different file type, mainly vector so it can be resized easily without losing quality.   Either you will have to pay extra for all the file types, or you will just be out of luck.

Now if all you need is a simple design for the web, it is a good deal.  But if you are looking for an image for your brand that is completely you based on your likes, dislikes, and audience AND all the file types for print and web, you will have to use a professional.  Basically, like with anything else, you get what you pay for.