If you are a highly accomplished, highly respected individual (like Linus Torvalds) telling someone 'that is just stupid' can be effective, because most people will instinctively trust that you have a good reason to say this and are probably correct.

But that's a special case, not a usual one. Unfortunately, quite a lot of people say things are stupid when they don't understand them (often because of an inflated sense of their own expertise). If they can politely explain why they think an idea is bad, they are more likely to be listened to, and they can save face if the other person successfully counters their argument.

Bottom line is, if you go around calling ideas stupid you better make damn sure you're never wrong, otherwise, well... that's just stupid :)