The workflow is inconvenient. Reliably executing a batch of jobs will carry a lot of overhead in launching additional jobs, continually monitoring your service quality, managing reputation scores, etc. If you don't expend this overhead (in time, money, and effort) you will pay for service you don't receive or, worse, will end up with incorrect results.
If its possible for people to run this thing on a fairly normal home computer then these extra overheads are not a problem, because compared to a commercial operation the 'miner' will have substantial cost savings due to not having paid for dedicated hardware, facilities, staff etc which should more than make up for the additional computational costs.