OK where to begin,
At the moment there are academy sides for all the "big" teams in England. These are all well and good for a player up to the age of graduation from academy (18/19). Then if they are not an exceptional talent they will go into the reserve league, which is a complete joke as it is full of players either regaining fitness, unwanted by the club or crap. This does not give these young players a competitive envrionment to play in so their development stops.
Say Chelsea (seeing as I've hijackewd the Villas Boas thread

) have an 18 year old LB who their coaching staff feels is good enough to play L2 football and has the potential to grow into a PL starter, they let it be known to the 24 L2 clubs that they have a LB with no first team experience at any level who is available for loan. 10 of the L2 clubs come back saying they are happy with their current LB, another handful say that they do not want to use one of their limited number of loans a season on a LB as they want to strengthen other positions, and another handful say they want to try and sign a LB with experience of playing at that level.
Lets say that this leaves 2/3 clubs who may express an interest in signing said LB, they then have the pick of all the LB's at all the higher division clubs who are available for loan, so for every one player who does secure a loan move there are many more who don't. Then on top of that the L2 is striving to be succesful in their own division, so they want this LB to come in and hit the ground running as they are trying to achieve their own goals for that season, and as I'm sure you can appreciate not every 18 year old will be able to do that, so he goes on loan plays a couple of times doesn't impress and is dropped/sent back to the parent club. With B teams they play in a competitive league but the parent clubs won't be as bothered about the results as they will be used to develop the players into potential first teamers.
Thats one way it helps off the top of my head, there are more but I cba to type them all out, I'll just respond to your questions/arguments as to why they are a bad idea.