I sympathize with those who are new to the SE "system" and aren't yet accustomed to all of the subtle differences between a Q&A site and a discussion forum, so I don't blame him for being annoyed over the downvotes.
However, such placeholder answers basically fall into the comments-as-answers category, and it's already well-established across all of the SE sites that this is to be discouraged.
Yes, a placeholder is going to be an answer. But it's still not an answer right now, and everybody should understand that the answer section is for answers - period. And the typical response to anything posted as an answer that is not actually an answer is either (a) a downvote or (b) a moderator flag resulting in the post being deleted.
It's not personal. But a casual onlooker has no idea whether or not that placeholder is really going to become an answer or when (what if the author forgot?), and in the meantime it's just sitting there, cluttering up the answer space and not really helping the person who asked the question. It may even be discouraging other members from answering, which is a Bad Thing if it's accidental, and a Very Bad Thing if it's intentional.
I can only think of two reasons why somebody would want to leave a placeholder answer, and those are:
To inform the question author that an answer will be forthcoming (leave a comment instead).
To remind the answerer to contribute the "real" answer later (star the question instead).
So please, don't leave placeholder answers - use the comments and stars instead, and if you see an answer posted as a comment (including placeholder answers), the best thing you can do is leave a comment on that answer similar to what I've said above ("This should be a comment; please use the answer space for answers only.") and flag it for moderator attention. If the author does not either fix it or delete it within a reasonable time, then one of us will.
From experience on Stack Overflow (not as a moderator), it's much more efficient to do this. Your instinct will be to downvote it, but that usually ends up being less effective than a comment and a flag.