Well, the musician is always the thing.  A really good violinist or fiddler could probably do some good sounding songs off a fiddle made from a cardboard box while I could still manage to sound terrible on some rare and wonderful antique violin. 

Practice and some degree of education are what will eventually make the difference.  I personally put education a bit lower on the list, since education is always just preparation for action.  Without action to gain experience, whether it is practices, jams or gigs, education doesn't accomplish much on it's own.  I am not saying it isn't important to learn stuff.  But I can spend an entire evening looking at educational sites about violin and even watching instructional videos, and unless i actually pick up the instrument myself and apply some of it, it doesn't help much.

All that being said, I do feel I did gain a bit out of those 100 reps.  It is no guaranteed "short cut" to getting good real fast, but it is practice.  Practice, especially consistent practice, is probably one of the biggest keys to getting better at anything.  Some note changes that were a bit challenging at first were easy by the end of 100 reps.  I learned some tricks for tone, like intentionally playing some notes closer to the bridge and some closer to the fingerboard.  I'm pretty sure my bowing got a little better as well.  So I would say that I found it worthwhile to do 100 reps of practice on a song, especially if it is a favorite song of your for one reason or another.  Just not over such a short span of time.  Spread out over a week or two weeks, it could be very reasonable.

Now, some of the attention to fine points kind of grew out of the boredom that set in at first with doing one song over and over.  You get a little bored and you start trying changing small things to hear how they are different.   That was an element that was missing from my practices before.  I changed to another song before I started to get bored.  A lot of boredom would be bad, but pushing into it just a little now and then, in controlled doses, might be good for me personally.

Now some of the talk here seems to be tending towards how better players wouldn't need 100 times for a song to sound good.  That may be true.  But I don't care how good I get on any instrument, I won't begrudge a good song 100 reps, at least over time.  If it is a good song, I could probably listen to it 100 times over the course of say 3 months and still not be sick of it.  On guitar, I never kept track, but there are some songs that I must have played literally thousands of times.  Some in practice, some in rehearsals, some at gigs, some because they are a favorite of someone, and some of those repetitions just because I love those songs.  They are just ones where I am likely to pick up the guitar or another instrument at any moment of the day or night and play for the love of it. 

So 100?  Bah.  A small price to pay for a song you actually like, to get it down a bit better.

I think Mad_Wed may be on to something with the idea of having something like a "page of 100" thread for those who keep track.  If Mad_Wed does get it going, I think I just may participate in that activity.