Tl;dr try someone out part time before hiring them full time.
Ignoring the link bait title of the post, I think the OP's suggestion is good. However, it's usually not possible. People are working full time and don't have extra time to work on your side project.
Interviews should include the same activities the interviewee will be doing during their job. Programmers need to program. Designers need to design. Sales people need to sell. It's actually quite easy to do this effectively in an interview and plenty of companies do that quite well (i.e. They don't suck)
Ignoring the link bait title of the post, I think the OP's suggestion is good. However, it's usually not possible. People are working full time and don't have extra time to work on your side project.
Interviews should include the same activities the interviewee will be doing during their job. Programmers need to program. Designers need to design. Sales people need to sell. It's actually quite easy to do this effectively in an interview and plenty of companies do that quite well (i.e. They don't suck)