And my first thought was.... well, no, not exactly. Or, well, not just (which is how that person made it sound).
The agent's job is to take their current clients' projects and get them into the hands of publishers in an effort to garner offers (hopefully multiple) and secure a contract for publication. And thereafter to continue by following up on payments, royalty statements, and subsidiary rights. And that succinct summation doesn't even begin to cover the amount of details involved. Or special cases such as what happens when a contract gets canceled, or a project gets orphaned (publishing speak for losing your editor due to a move or "right-sizing").
And, if that agent feels they can support additional clients, part of their job would include looking for those.
I also personally see the advantage of being an advocate for the writing community in general. Hence, blogging for the last several years, attending conferences to deliver workshops, etc.... Participation in the conversation of writing and the craft, not just the business side of things. For me, there's more to it than just closing the deal. I find it's healthy for me to connect back into that gestalt and not just let it turn into pushing paper (virtual or otherwise).
Back to the original comment that spawned this -- where does reading non-client submissions fall into the scheme of things? It's R&D. A good company realizes that it will need to grow and evolve even if it has sure-fire products that currently satisfy the market. An agent needs to do the same thing. But R&D is a projection into the future. It's often challenging to express it in terms of tangibility, at least at first. It's based on experience that it pans out and faith in choosing the direction that will do so. So, queries are inherently speculative. Luckily, we don't have to justify the time and resources spent on them to stockholders....
But it's still time and resources. And a balance has to be struck. Which is why it seems some agents don't see reading queries and new submissions as their primary work. This doesn't mean they don't think it's valuable. It doesn't mean they don't want to keep looking.
There's nothing quite like a break-through discovery. There's a hunger that happens for something new and shiny. But there's also the drive to build and develop something lasting and good. I have clients who are brand new; I have clients that I've represented for more than a dozen books, from their debut to the first book to hit a bestseller list several years later, and being involved in that is also an amazing experience. Adding to this can enrich the entire spectrum. Which is something to keep in mind. But it is hardly the only thing.