I think that, apart from simple snobbery, the contempt regular soldiers often have for irregulars comes from having seen or heard of them breaking and running in face of an overwhelming attack, whereas the disciplined regulars tend to hang on longer, sometimes long enough for the tide of the battle to turn. Usually when the irregular forces crumble like that, it's because they've been put into an unsuitable situation through the incompetence or bad luck of their commanders.

Not surprisingly, throughout history regular forces have usually defeated irregulars in situations which give advantage to their discipline, training and weaponry; irregulars win when the situation gives advantage to their individualism, knowledge of the local environment, ability to travel quick and light, and their weaponry. Often it seems that difference in numbers aren't as significant as these other factors.