In many European leagues (not sure about France) the top clubs have reserve teams playing in lower divisions which allows them to better judge so called fringe players (and keep them match fit). In Portugal, Italy and the Netherlands this is possible at level 2 of their league - so Jong Ajax, Jong Utrecht, AZ Alkmaar and PSV all have teams playing in their second division. In Germany and Spain it's possible at level 3. At Ajax they are required to register players as being in the first team squad or in the Jong Ajax squad in the second division - with change over only being possible during transfer windows, or if a player has not yet completed 15 appearances for the first team. On the negative side Jong Ajax cannot be promoted, even if they win their league - but this ensures that new players to the club can be thoroughly tested, at a competitive level, before moving up. Clubs like Bayern Munich and Borussia Dortmund have the same. Unfortunately no such system operates in England - but my feeling is that if a Watford 2 squad were playing in, say, division 1 that it would have far more of a positive effect than the occasional appearance against other under 23 sides. Most leagues allow this so why not England ?
The set up in England is very different to other European countries. There are five professional leagues in England, with many of the smaller towns having a team and a dedicated bunch of supporters. In France there is just two fully professional leagues, and the third tier is made up of teams which will have some full time players, some part timers and some amateurs. The teams in the two professional leagues do have second teams playing in regional leagues, but not above that level.
If the larger clubs in England were allowed to have second eleven teams inserted into the league structure it would push some of the small town teams into a lower and probably poorer situation than they are in now. Since 1958 when the old third divisions north and south were merged to form the fourth division, and since 1987 when the conference had an automatic right to promotion to the football league, football has become more national and less regional. While that gives teams the chance of a rapid rise through the divisions, it also creates problems for those going the other way. I still want to see the fans of the small clubs have their day, and don't fancy the larger clubs who have all the money take even more out of the game with second teams in the leagues.
