Bumping, with "bump", obscures worthwhile discourse.
However, commenting in threads which deserve more attention has all of the merits discussed above.
What could we possibly do about this?
How about "bumping" threads by adding actual content to them? If a question remains unanswered, how about rephrasing the question or adding detail to it to turn the whole thread into something which might do better at sparking a discussion? If a topic warrants further thought, how about typing out the questions that it seems to warrant further thought about?
If you simply feel surprise at a post not having been discussed better, why not write down what surprise it causes you to experience and why you would have expected it to be more popular?
If the only thought in your mind upon seeing an old post is "bump", the post is likely not worth saving.
If a post inspires thoughts yet you cannot be bothered to write them down, posting a non-comment on it is worse than useless.