It wasnt worth it back in the day as usually you would have to put up with a large police presence. Used to just get crap though, not sure if it has been updated like the RBA or Fuel station robberys.
The AR Weapon Cache used to be a police armoury essentially at least in RP, the building was never used by Police and was something rebels to rob and "have their fun" with by attempting a robbery of the building. To gain access to the building you are required to have purchased a Industrial Code Breaker from the one of the Rebel Outposts at this point in time I'm not entirely sure of the cost however but it will set you back. At the moment it has changed when you break into these crates you will receive a virtual inventory item which we be listed as a "Weapon Cargo Crate" in which is then taken to Money Laundering on the Map where you exchange these for money. Also don't forget your trusty bolt cutters to get into those dusty crates inside the building.