Come on mate,you know what i mean. Nobody in their right mind will go around sneaking in full armour. The most important thing for sneaking is firm legs,staying crouched for some time is harder than people think.
Indeed. So when you consider the rogue by my definition above, it's not a class who's about sneaking and delving in silent lethal endeavours. And it becomes perfectly fine to build him with heavy armor. Like in the case of a scumbag knight etc.
Also, I didn't mean to say that it's forbidden to sneak in plate armor in ad&d (like, for multiclassing thief/fighter), I expressed myself poorly there. I mean't that plate is forbidden to rogues by default, because yeah they're about sneaking and shit.
If you want to have big sword and plate armour. why don't you play as fighter brigand or something?
Rogues are where the flanking bonuses, blinding strikes etc are. Ie the scumbag tactics.
Unlike what's been said, classes are not interchangeable in this regard. A class might fit a concept better than another.