I think it depends on the project - if for a once of project or something that you haven't bet the house on, that sort of thing can be good. However, you don't get a lot of the benefits of working one on one with a designer, and may often get a lot of half-arsed work from people as they don't have the ability to really understand your business and design requirements like someone working one on one on project can. I wouldn't use them for a major branding of business you were trying to take big.
Also make sure you get all the versions you need in all required formats at the end of the project, because designers on these type of systems won't generally give you any "after sales service" that a traditional designer will.