Indeed its not hard to imagine how "uber for day laborers" could increase worker exploitation. In particular the possibility of somehow skirting around worker's comp insurance by arguing that it only facilitates transactions is an obvious starting point. However, I think such a technology could also be used to mitigate exploitation. The one thing that I already mentioned was payment assurance. In general socializing the risk exposed to laborers as part of the service could be a great benefit in growing the service.