- Someone injects an obviously inferior idea for political reasons (e.g. the recent TLS 1.3 debate). Then, a rude response might be your best option to deter the opponent. I think, some of Linus's attacks from the past fall in this category.

- Someone is just inexperienced and proposes a bad idea with good intentions. Then go for factual "This is flawed, here is why".

- Someone proposes an idea with flaws, but you see potential beyond the current proposal. Then, one should politely point out the current weaknesses, and at the same time encourage the proposer to improve on it.

As successful leader, you need a repertoire of responses.