Yes, for a BAD artist it's never done. I've took many an art classs in my day and at one point I was thinking about making a career out of it. One of the first things they drill into you is not to work your art to death. You'll always have that feeling of wanting to do more, but you are supposed to resist that urge and let it go when you find a good stopping point.
A mame cab is not art, it's a craft. 
I'm sure both good and bad artists produce both finished and unfinished pieces or does finishing constitute a virtue of a good artist only? Sure, knowing when to call it a day is a good thing but it's such a subjective and personal assessment. By that I mean one persons "working it to death" is another’s "barely started". I think somewhere in that statement about how long to spend on something is also the sage advice - Ultimately it's not about how long your cab takes to build, but about how pleased you are with the result.
It's like this... good art is subjective, but bad art isn't. That's not very fair, but unfortunately no less correct. If 90% of the population sees bad art, they generally all agree that it's bad. Bad art has merit, but I don't think anyone intentionally sets out to make bad art. That should never be your goal. We aren't talking about how much you rework something, we are talking about the fact that you HAVE to choose a stopping point and stop. This isn't optional, it isn't subjective. The purpose of art is to invoke a repose from your audience, if you don't finish then that can never happen and your art is a failure. Aka bad art.
This is another thing that I got flamed for (for no reason btw) a few months back, but nope, how the artist feels about the end product, in regards to it's quality is largely irrelevant. It's how the audience feels about it that's important. If this isn't your philosophy in regards to your work (like cab building) then it isn't art. So like I said, mame cabs aren't an art, they are a craft.
