I do believe that is better to answer to these emails as much as possible. At the same time, the answers do not mean a compromise with the person.
And I agree with Joe that the role of the recruiter is to find people for the jobs and not the other way around.
Nowadays with all the competition we have on the market and with all the demands that the companies have on their profile, is somebody is looking for a job should put a bit more effort into writing a job request email I order to catch the attention. Just “looking for a job” is not enough.

I personally try to respond too but it depends if the individual has applied an element of etiquette to their enquiry. If they've bothered to find my name and explain why they would be interested in working for my company then they can expect a reply, but if their "I'm looking for a job" message feels a bit spammy then I'm inclined to ignore it. When I trained Recruiters, I would explain to them that they need to be ruthless with their time if they want to succeed in the industry - replying to non-appropriate candidates wouldn't fall into the 'time-well-spent' category. If I was coaching a business client, I would also say the same, be ruthless with your time and don't get reactively pulled into activities that you haven't scheduled for that day. I think there are many better ways to find a job that sending a random "I need a job message". Job seekers need to have a strategy and that's at the heart of what my company helps people with.

I rarely get "cold" emails like . you describe. I do get a lot of emails/calls from people who were recommended to me by someone I know (either well or only vaguely). I help everyone I can. I think that I am paying it forward so that when my daughter is out of school and looking for a job people will be considerate and helpful to her.
The statistics say that most people in the US get a job through a connection--not from requisition/ ad on Linkedin or Indeed. So, helping is the right thing to do

Like Matt, I pay attention to the tone of the email received. And, like Ellen, the "warm" mails are treated differently.

For the most part, I respond even if to just say thanks for sending your information.

Our recruiting is quite focused since we're in Japan. Mainly, it's focused on bilingual Japanese, but we do place non-Japanese into positions, so I don't like to just ignore such emails. And, I believe the courtesy of a response is a good thing to do. If it's too much work, just use a template at least.

Finally, it may sound a bit "cold" but the reality is, the job of a recruiter is NOT to find jobs for people - it is to find people for jobs.

If it is only a cold "call" , via e-mail, I assume that this went out to hundreds of people and I do not respond. That said, if it is a person who is a current Linkedin connection or someone who is connected to someone I know, reaches out, I will always respond. I recommend that the subject is not "I need a job" as this is about solving business problems and they can be solved multiple ways (consulting, project work, full time, etc.) It should be about expanding a network which would encourage more people to say yes.