Teachers probably are just trying to ensure their students actually turn completed works in on time.
It's a good midset to try and develop, though, eventually you just have to let go and say it's ready for production, even if in your heart you know you the colors are not exactly perfect.
I especially find this is the issue with writing. I've heard a few writers say they never go back and reread their published works because they will find themselves wanting to change everything. I suppose this isn't impossible to do, I've seen reissued books, but most readers don't take to well to revised works unless the changes are not significant, or the book is added to rather than changed. Digital visual art is a lot easier to change than any other kind of visual art, but if I don't restrain myself, I'll just work on the same picture my entire life rather than ever starting anything new, heh.
This particular piece wasn't commissioned, but my portfolio is heavily weighted with ad design, web design, brochures, logo design and the like. I'd love to try and get some work illustrating, so I'm working on developing those skills and fleshing out my portfolio.