Fubaar... In the specifics you now describe I agree that it was not a great experience and a lot of experienced F/Os and Captains were unfairly treated because of it. I was training new DECs just like you describe and they were earning more money than me! However, I am sure you would agree all DECs are not the same and that it is at-least reasonable to expect the guys in the cockpit to help each other out where and when the may need it.
We will probably all end up as DECs for some airline at some stage of our career and hopefully the guy's job you've taken and are now flying with won't be to pissed off.