I think accommodation is the crux of the matter - if you are relying on pre-booked accommodation then you have lost a lot of the ability to be flexible with your route - some degree of pre-planning then makes a lot of sense - it really comes down to how much you pre-plan - getting to the detail where I have every road and junction listed/GPS'd and having looked at them on Google Earth to determine their pros and cons is pure anathema to me.
I have to agree - and sticking to hotels rather than camping makes it more complicated. We'd initially thought of booking every third night, and being flexible in between - but when we came to plan the route, even in broad outline, we discovered that a lot of small places had just one hotel (or none at all) so we changed our minds and booked the lot, for safety. As it turned out, in some places we were the only guests (in June) so we could probably have taken the chance, but we didn't know that!
It also had the benefit that it kept us on schedule in grotty weather - there was no discussion about cutting a day short and hoping to catch up later, we just got on with it. Since we were on a fairly tight schedule that ended up being a very good thing.
We did plan in quite a lot of detail, but didn't necessarily stick to the exact route if a better option turned up. We didn't use Google Earth - though I wish we'd known that the tiny little yellow D road into La Rochelle is the favoured route for heavy lorries. 2 miles of sheer terror!
In the end, it's all according to taste, and how much you like winging it.