There was another time I was planning to go to Europe for a major sporting event. That was before the 2006 World Cup in Germany. My goal that time was to see two or three matches and do so on a shoestring. I had scoped out two cities that had games relatively close together (both calendar-wise and map-wise) to minimize the cost.
I put quite a bit of planning into this trip and had an itinerary figured out. I was going to fly into Munich and see a game there, then travel to Cologne for another game about 2 days later, then back to Munich a couple days after that for a third game and then my flight back home. I didn't make any hotel reservations, but I did put my name into the drawing for tickets to the games I'd targeted. Unfortunately, after two rounds of the lottery, I never got picked for tickets.
As I was trying to do the whole thing on a budget, I didn't want to pay scalper prices for tickets, and since I was not going to be able to get tickets through the normal channels (and might not be able to get tickets at all), I scrapped the whole trip. I just couldn't make plane reservations without knowing that I'd be able to attend the matches in the cities that I wanted to be in.
One good thing about the TdF is that there are no tickets: show up on the side of the road and you're good. In many ways, that makes this trip easier to plan than a World Cup trip.