I agree with the general position that excessive politeness can harm the quality of communication. But I strongly disagree that the harm is due to there simply being more words. The harm is due to the presence of actual ("white") lies.

The prescription that seems to flow from your analysis - "Don't waste words" - strikes me as a bad direction to go. I fear that our comments and criticisms are often already too cryptic and confusing due to their terseness. I would advise people to use more words: provide a second example to clarify, quote the passage you are critiquing, explain the point of a link. As the saying goes, words are cheap. Trying to be frugal in their use is false economy.

My advice: aim for maximum clarity. If you are considering adding some polite words simply to soften your criticism ... don't. It damages clarity. On the other hand, if you are considering whether to prefix your criticism with "I liked the first part, but ..." then go ahead. It clarifies the scope of your criticism. Even though it "costs" some words.