One reason why the progress bars don't progress at an even speed is that they are meant to indicate 'real' progress. So, until a task is finished, the progress should not be visualized... however frustrating that may be for the user.
One of the things I've noticed is that progress bars are more task oriented than goal oriented. You can see one file being copied and then another... this is not useful as it doesn't give a real indication as to how long it will take to finish the goal.
A goal oriented progress bar, on the other hand (in the way that you propose) is sometimes very hard to achieve as some subtasks could be optional.
Having said all that, I do like your idea, but the reality is that the world is full of incompetent (ups, did I say that?) programmers/analysts/consultants (or lazy, or both).