They are all harmful to the graphic design industry and should be avoided at all costs.
When participating in these logo contests, you are devaluing the design industry as a whole. Also it gives off the perception that logos can be "ready made", with the ability to simply change the name and have it work. There is a LOT of hard work, research, and unique consideration that should be taken into account when creating a great logo. Even if you decide that you would like to work for pennies, think about the greater implications of bringing down the industry as whole.
Example, "Why would I pay hundreds or thousands of dollars for a logo from you, when I could go to this site, have 50 people make me different logos, and choose one for $100?"
That is the point of view that people begin to view the graphic design industry with after seeing or using these contest sites. Of course many will be able to spot the difference in quality and understand that you get what you pay for, but at the same time many will not.