First, I'm surprised how little respect design gets in the tech industry and from programmers. Good design can sell a poor product or inform consumers about a superior product. It can drastically reduce support costs (UI design) or encourage consumers to pay a little more for a product (i.e. The Apple "Tax"). Design is important to the overall success of most business ventures and should be budgeted for and planned for accordingly.
These logo (I'll be nice here) marketplaces are better than design-it-yourself or no design at all in most cases. But you get what you pay for. Many of the designs will come from students that have little to no experience and an incomplete grasp of the principles of design. Most of the rest will come from international locales far away with different cultural identities and little understanding of yours. After all, a logo is not just something attractive, it communicates a message visually.
What you will get in abundance is poorly thought-out concepts, logos stolen or similar to others and rushed work with a total disregard to overall quality. Does your logo need to be legible from far away or at small sizes? They probably didn't account for that. Some of these logos will land you in court (trademark/copyright violations) and I guarantee the guy that "designed" it for you on these website will disappear.
Bottom line: These logos are great for a placeholder until you can afford a designer, until your product grows or launches or for presentations to investors, etc.