Euroboy
A plan is only as good as the current situation. A change may be passed onto a crew member in the honest belief that it is the best way to cover the flying program with the available crew. However, enter another sickie, another tech delay, another wx diversion into the equation and the plan goes to a bag of cack. Also, there are few computer systems on the market that will track a crew members changes, and even if you have such a system it is often to long winded to go in and find out. You will often get a situation where two crew get changed onto each others duties over a period of days. Add to this the complexities of working with multiple aircraft types and crew and the problems very quickly become insurmountable. I'm sure that any crewing bod will tell you that they do not do changes on a whim. Every change you receive has been caused by any one (or more) of the reasons already stated.
Put your name forward to your manager as a volunteer to spend a couple of hours in crewing, preferably during a summer weekend. I'm sure you will be unpleasantly surprised at the problems that are faced.
Likewise, I would recommend any crewing bods spend some time in the crewrooms and, if poss, on fam flights. Getting to know people goes a long way in dispelling animosity.