If it's a comment left by someone who you have a relationship with, then it is probably courteous to respond, unless the comment is clearly a no-replies-needed sort of thing.
If it's a comment as part of a conversational thread by some people who are talking to each other (and they just happen to be in your space doing so), then your overt presence in the thread can be taken as intrusive and ... over-attentive. Like the party host who attaches to a pair who are getting to know each other better and butts in.
If it's a comment from someone with whom one has no current relationship, I view it as entirely up to the host whether they respond to it or not. It is generally nice to, but I view that as "it is appreciated and gracious, but not-responding is still a socially valid option", rather than "not responding is rude".
This is especially so when it's a comment from someone who has never interacted with you before and they don't say where they're coming from, or part of a storm of comments originating from someone else's link to your journal, particularly when it's a link from a source that you're not involved in, or a source that's actively hostile. If your entry has just gone viral, you are absolved of all responsibility for responding to every damn thing that J. Random Internet User has to say about it. If you're a professional, sometimes you have to choose between actually doing what you do, or replying to all the people trying to talk to you. (Though a mass-response, like a new entry with "Hello to the entire internet who just dropped by to comment; your support is appreciated, thank you" is nice.)
Let me say that a little louder: a person contacting me does not oblige me to spend my time on them in return. Their desire to create a relationship, even the tenuous relationship of a mutually commenting interaction, does not oblige me to give them that relationship. I'm not obliged to respond to spammers, I'm not obliged to respond to people adding me on Twitter, I'm not obliged to respond to people I don't know saying "me too" on an entry of mine, I'm not obliged to respond to someone trying to troll me or start a fight.
I can speak in public without committing myself to acknowledge or respond to every person who has something to tell me about what I just said.
If I hampered myself with that restriction, I would never have posted any of my widely-linked entries in public. I make an effort to respond to most reasonable comments, and I apologize if it takes me longer than I think necessary -- but I would never make an absolute commitment to respond to all comments, I would never endanger my health in order to respond to all comments, I would never let responding to all comments stand in the way of my actual duties or getting the stuff that is in my head out of it.
It's also different in communities in some cases; things like dw_suggestions or suggestions are going to feature people coming by months and years later and adding to the discussion, and saying the same things and no, you're really not obligated to reply to every suggestions-commenter ever. The community's maintainers/administrators are the ultimate host in a community.
It is nice to maintain a consistent framework for replies, but it really varies on what you're using your journal for, and whether it just exploded because the entire internet dropped by.