Also, lots of companies have clauses against this in their employment contracts, or close enough, that candidates wouldn't risk it. Anti-moonlighting clauses and clauses about IP ownership, specifically.
This one is big and so often ignored. I am breaking my employment contract if I code for money or if I write code that will be used for anything. I have to get written approval for contributing to open source. Do you, as a potential employer, want to start our relationship with me violating employment contracts?
We wouldn't want someone to violate any obligation. When we offer the trial period option, we always have a discussion about whether that's something they're able to do, and comfortable doing - work-wise, family-wise, etc.
