Does every project actually need to be brought to its conclusion? No. Is it nice to take pride in completed projects? Yes. Do the projects I do bring me additional knowledge? Yes, usually.
I think that the ability to complete a project is a desirable skill. I think the desire to take a project to completion is worthy, otherwise why start it? Still, priorities change right along with interests. Much like saying "No" is considered a skill, knowing when to finish, and when to throw in the towel, are both equally valuable skills.
When I signed my daughter up for soccer this summer I had to willingly drop a personal and extremely fun side project because I've committed an extra 4+ hours/week to her development. Am I not a grown up for making this decision?