We talked about many different way to present ourselves as professional designer, in order to make lots of money. Producing Spec Work was not one of them.
Spec work does not help anyone, because no one is fully invested in the project. If you need help saying NO to a potential “client” that tries to convince you/ pressure you into doing spec work, read this article from AIGA: it will give you ammunition, and also a sample letter you can use to refuse.
If you need more convincing, check this article from nuschool.com that details why free work is not valued.
For fun, watch this short video, created by TopicSimple on what spec work is: