BBF,
The reason that 'via delta line up RW27' is used, is purely for safety reasons. There have been several incidents where ATC thought that they were lining up an aircraft at a particular point on the RW but the aircraft was actually holding at a point further down the RW and started to line up in front of the previous landing aircraft - scary stuff! Also it helps prevent other aircraft from taking your line up call by mistake. Finally it is good for the situational awareness of all crews on the airfield. In my view it is certainly not wasted airtime. Hope this helps!
Regards, ADIS