mister geezer
Promotion should be on merit, not length of service
You are partly right. Merit is only one part of the equation. Length of Service ie loyalty should also be rewarded.
How would you like to be somewhere patiently waiting to amass hours and a command only to be pipped at the post by someone who joined last week with 500 hours more than you?
How, in this case, do you define merit? Hours? Types on licence? Education? Background?
Also, a lot of scope for cliques to promote their own eg Air Force giving their own guys a leg up, Aussies giving their own a leg up, ex-Oxford cadets giving their own a leg up, ex-BA guys etc............
Get the picture?
(Incidentally, I have nothing against any of the above groups - just using them as an example.)
I'm afraid, despite what many think, Seniority (LOS) is still the system least open to manipulation. Let's not forget that merit is still a very important part of the equation. Without it, Seniority doesn't normally work.