Every time I have wired up Panasonic PABx's there has been at least one one that wouldn't work without it..Its surprising but MOST 'commercial' phones still do have the ability to do loop disconnect..usually a switch underneath..and the cheaper ones are pared to the bone and don't have internal capacitors.
Since the main reason to USE an analogue compatible PABX is to get the costs down - with a typical analogue phone at 20 quid or less, but a key phone at 60 quid plus, you can see why - it makes sense to equip all the outlets with master sockets.