The title says it all, sometimes a person posts a really lame comment or even one that is just plain wrong and misleading. Why can't we downvote those just like we can download answers?
Because they're just comments.
Comments aren't supposed to contain meaningful information. Their purpose is to ask for clarification, point out something the author should maybe add, remove, or edit, telling the OP he's a moron and why, explaining that we can't guess what you're babbling about without a schematic, etc.
Of course comments get abused, which is why you want to be able to vote on them. However, the real problem here is that someone is trying to use comments to supply information, not that you can't downvote them.
Since comments can't be downvoted and are rarely policed, they are useful for, but not intended for:
- Saying something unpopular you know would get downvoted in a answer.
- Providing a quick answer that would otherwise require too much work to write properly, and that therefore would likely get downvoted.
- Providing a answer that you're not sure is right.
- Saying something off topic, possibly humerous.
Put another way, if you can't beat them, join them.
Comments do sometimes get cleared out by mods, so don't spend a lot of time on any comment. Of course the character limit pretty much enforces that anyway.
Google fu is weak
google magic: comment downvoting site:meta.stackexchange.com