In my experience, candidates with major pet projects, especially ones that aren't related to their current job, fall into one or more of the following categories:

* They don't have life, and are usually not the most pleasant people to work with, let alone communicate with non-programmers.

* For some reason cannot find a job that involves the passion/knowledge/skill they put into their pet project. The job market for programmers being what it is, that's a major red flag.

* They have a passion for programming, but aren't good enough to hack it in a professional environment. Often not so much a skills but discipline or team player issue, but yes, you can have a passion for something you are not particularly good at! Millions of amateur-[enter activity here] do it every day!

Having or not having a pet project is an good angle for an interview, but not a meaningful qualification in itself.