On the worker's side, in addition to the 10 % fee there are also e.g. fees for money transfer, so oDesk earns on the interests for deposits (though that was recently limited). The system and restrictions for entering hours are painful but WMF is reportedly never using the worst sides of them (like the application which takes screenshots every ten minutes). I don't know how their costs compare with competitors', but as far as I know some were considered and one candidate company identified; the WMF also has some full-time employees abroad who don't go through oDesk, so for some countries WMF can compare the administrative costs. I've no idea how big a part of the savings allowed by using oDesk consist in an erosion of employee benefits and other legal/contractual advantages the worker would have if paid/employed directly/locally. It would be useful to set up a Meta-Wiki page listing the basic requirements WMF has for one such service. As for other employers on oDesk, yes: even though the only "official job type" is the keyword "mediawiki", it's trivial to find dozens of obvious paid editors in oDesk with a simple search; the subtle and really dangerous ones will make their profile private and reveal themselves only to job posters. However, this matters to the WMF only in a minimal way, namely that WMF employers can use "points" earned with WMF (billed hours and "stars"), shown on their profiles, to facilitate other jobs: the same they can do with LinkedIn's work history and recommendations. WMF probably wants to ensure that such feedback/recommendations from WMF are given fairly but I doubt it's a big deal. On a more practical and less formal point of view, it's possible that the place exposes workers to "offers you cannot refuse" (and "l'occasione fa..."). If this is the case, simple countermeasures may be found and suggested to contractors in the handbook to compensate human weakness (e.g. disabling job offers?).
Nemo