fishtank, growing a pair is your answer to being messed around. When you are a new starter and a reserve/floater, you can get messed around by the definition of not having your own round; they use you for all sorts.
Are you really a delivery postman?
When you do a round over a period of time, you defo get better at it and finds ways of saving time compared to being put on a new round or rounds you are not that familar with. A lot of experienced member of staff in my office would not even know where to begin if put on a round they are not accustomed with.
Note: some offices have merged and some postmen have longer to travel from their office; and very often they are the new guys.
Consider simple things like redirection: when I had a training round over a 6 month period, I could do the redirection as I went along because I know where they were. And also misaddressed letters can be solved easier with knowing the rounnd - and I am not talking about years of experience here...
With holidays, I was talking about being able to plann your holiday to the smallest details. You can plan your holiday with long weekends in mind and bank holidays and so on.
* To original poster, I sympathise but your manager is also right about distributing the pain of being messed around amonst the staff rather than the new starters that are already under pressure in all sorts of ways.