An added benefit of paid over unpaid assignments is that good candidates always have offers of work and are generally quite busy. There are quite a few “interviewers” who require you to carry out free work for pre-interview screening which can be elongated. Many of these are outright scams and seem to be a way for recruiter to get ideas for their work for client use.
I was once tasked to put together a presentation for the recruiter with regards to one of their client’s ad campaign. This was pretty open ended and being a perfectionist and facing a project without any sort of depth or length cap it could take up a huge amount of time. So I just backed out.
An unpaid assignment for the selection process will put off good candidates as it would seem like a waste of time.