It would be based on the following:
- Has to be good and mature enough for our first team to stay, and isn't judged to be (unlike established players or experienced squaddies, even if mediocre)
- Has to be shifted for a maximum return if he isn't good enough (unlike younger players)
- Has to play regularly to find out whether he stays or goes, and that can't be guaranteed at Arsenal
- The overall deficit of defenders in the squad isn't judged to be dire enough to hold onto him despite 1, 2 and 3
You quite often see potentially useful squad players like Chambers go out on loan for these reasons (Wilshere is another good example).
You could argue that such a policy is unwise, but if it isn't pursued at least relatively consistently, the footballing or transfer value of a lot of these journeymen gets wasted.
At the end of a loan at Fulham we hopefully either know whether Chambers is worth the investment, or ship him off to them or another PL club for a fee. If he stays at Arsenal we probably don't find out, and the uncertainty of the last couple of years continues.
