My understanding, and this is how my instructors explained it is that the extra "behind" is to reiterate to the sometimes too eager pilot that his is not to line up immediately. IE: the "behind" is the first and last thing that the pilot hears.
To me this makes sense. It is a conditional clearance, but in a busy port it means that you can give an aircraft instruction and move onto the next item. Pilots should be aware of jetblast etc and ensure that there is a safe distance between them and the departing aircraft. How many times have you had pilots call ready halfway down the taxiway? You know full well that he ain't going to make number one to the bus on final or that there is someone on the ground ahead, but it gives them a warm fuzzy that they can get onto the runway and stops them bugging you for more (most airports don't have a punishment bay for pesky pilots
) and it means that it is one more thing that you don't have to think about.
I don't think it is a particularly unsafe practice, providing you get the readback "behind".
Admittedly I haven't had the pleasure of working with a large number of international pilots - yes we do modify and slow things down for the ones that do come through. But the local - and I don't mean local to the airport, are perfectly conversant with what "Behind" means and are probably grateful that they don't get a hold short. I guess it means they can adjust their taxi speed rather than stop their taxi.