"If an application needs to access file space it should do so in a space allocated for that particular application or at most a class of applications."
The problem becomes, at some point, everything has to be done at once and, for whatever reason, you need a skeleton key: a last resort. Kinda like how the guy with the red key gets locked in a room that only accepts blue keys and the guy with the blue key gets locked in a room that only accepts red keys. Or in your case, someone's trying to install an application but needs more space to do it, but the user who cleans up dead space isn't available or is for whatever reason locked out. Too many locks, you run the risk of a lock-IN.
Furthermore, what if the application in question requires absolute speed (which is increasingly demanded to avoid getting complaints or--worse--getting beaten to the punch--think a trading floor).
Biting the hand that feeds IT
