This sentiment is what GP points out is cultural, not practical or factual. If you compensate them well for the hours they work, many part time devs will have superior per-hour output to FTEs and also be grateful for the added flexibility you afford them.
We hired a stellar part time dev recently (contract hire to start, we just increased his rate and verbally stated we are interested in continuing the relationship long-term if he is, and offered written agreement if he is interested), and he has been working out great. The option is open to him to work full-time whenever if/when he wants by planning it ahead of time with us, but our default expectation will remain being part-time so he can pursue his game development on the side.
Finding a great part-time team member is no different than a great FTE, once you and the employee are both free of the cultural negative baggage associated wtih part-timing.