Sorry - i beg to differ slightly...
Uniforms serve several purposes - discipline, uniformity, image, and rank/role.
Why do trainee doctors where white coats?
Why do training nurses / physios where their uniforms?
Why do training flight attendants where uniform?
(when i say uniform i mean some smart matching outfit of some generic description)
I think the answer to all of these is the four reasons i gave to varying degrees...
1. Discipline - This is more military but in the civilian world ironing a shirt, shining shoes require discipline. This skill is required for airline flying - so whilst training this is a good skill to develop and a good way in which to develop it.
2. Uniformity - Someone posted earlier that uniform takes away worries such as "what will i wear today?" type concerns and instead puts everyone on the same level socially. How many problems do people have in society worrying about their brand of shoes or handbag or who makes their shirt etc... There is no room for that type of thinking in aviation and uniform ensures that as much as is possible this type of thinking is suppressed - leaving people to concentrate on flying the plane.
3. Image - Aviation as a whole is considered a serious business. I'm sure anyone participating in this thread knows someone who has asked them "wow - how do aeroplanes fly? thats so amazing etc etc etc...". Lawyers are expected to appear smart, doctors smart or white coats etc, accountants and most professionals are expected to turn out smartly dressed.
How would any of us feel if a lawyer turned up to defend us dressed like a tramp?
How would any of us feel if a doctor turned up to treat us and looked like he/she hadn't even bothered to look serious?
We would feel a little uncertain of their ability to perform their roles - wouldn't we? (that is fair enough).
So image in this sense is to do with the expectation of the general public / customers / and anyone who looks at aviation to see people who at least look like they know what they are doing, and appear on the outside to have more perceived confidence in their ability.
The general public naturally have a sterotype mentality - they see someone in a double breasted suit, well groomed and they think either - lawyer, banker etc etc.
In the flight training environment - i think wearing a uniform adds to the natural confidence of students, makes them take pride in their appearance and provides the right image and confidence to those people paying for it.
4. Rank / Role - OK lets face it aviation has its roots in naval/military settings. Why do we have captains or first officers at all? - Simple because their needs to be. CRM/MCC may try to reduce the authority gradient in the cockpit so that junior FOs don't feel so uncomfortable with Senior Caps but at the end of the day there is always going to be Rank & Role.
It is simply unacceptable for crew/personnel to say "which one of you is the Captain" because the chain of command should be clear and unambiguos at all times.
So using bars is the military method of rank ie. more gold/silver means more authority... it's simple and works - people know their place and can get on with the actual job of flying the plane etc.
In the flight training environment the question of "is this nescessarily good practice to echo this airline policy of rank/role?" i think can be answered with Yes - because - you are also training to work with the rank and role structure.
Think about the amount of people in aviation who have an attitude problem. They have never done military service or experienced working within a discipline / rank role environment - is the time for them to learn when they get to the airlines or is it best to remove the ruts during training?
I think you probably know the answers.
On another point with regards to professionalism and uniform...
I think professionalism is not just how you act but also how you look as well.
To be professional you have to..
Talk the talk and walk the walk AND look the business.
How seriously would you take a pilot who turned up to fly a passenger transport aircraft dressed in jeans and a t-shirt?
They may be the best pilot in the world but people are going to automatically think first off - "Oh my god THAT is flying our plane!"
Aviation is a serious business. The people involved in it have to act serious, think serious and look serious. This applies both to operational aviation and aviation set within a training environments as well.