Say a post has been inactive for a period of time, what happens if you answer, comment or even vote on that question: who sees what you've done? Is voting good practice on inactive questions as it keeps them 'alive' so to speak. Is the OP alerted if any of this occurs. I've read this 'bumps' the question back onto the front page, does this happen every time one answers, comments or votes on an old question?
Answering bumps the question but (I think) only for those who have clicked the "active" option, not for those who are using "recent". Commenting notifies the person who posted the question. Commenting on an answer also notifies the person who posted the answer. Commenting may also notify a few others who have contributed to the question/answer – see the related posts about comment notification for details. Voting only notifies the person you have voted on.
Also, anyone who stumbles across the question at some future date benefits from your answer/comment/vote, and I think that's the main point in taking these actions on old posts.
My first post was quite recent, answering an OP that was about one and half years old. It got an immediate response which was quite gratifying and made me feel welcome on the site. However, my post was more of a comment, though I posted it as an answer, not knowing any better. Apparently I would not have bumped the question up to active if I'd followed the guidelines and just commented. But then, my 'comment' ended up correcting a mistake that resulted in a better answer. It's possible that many users that came across the old discussion were confused by it. So, if anything, commenting on/answering old posts should be a priority!
As noted, commenting (resp. answering) serves as a "message": you can notify an answerer/commenter (resp. OP) of your thoughts on the question. In this sense, there is certainly a point. Don't worry much about bumping as the bump is very temporary and is not caused by comments. As far as votes go, there isn't even a counterargument, really. It requires little effort on your part and doesn't bother anyone (unless you're downvoting, in which case standard etiquette applies).
There is also another thing to keep in mind, if you want your comments or answers to receive significant attention: is the question popular? If it's popular, your contribution, in the near or long-term future, will likely receive substantial views. On the other hand, more obscure posts may not; indeed, perhaps you may never receive a reply or a thumbs up.
You should still post, however. Certain topics require specialized knowledge and hence are less popular. However, twenty years down the road, maybe a graduate student is working on a complex problem in a specialized field and stumbles on a helpful contribution of yours. In this situation, posting a comment or answer for this one hypothetical graduate student would be reason enough to post.