You are an attending? Then likely your kid isn't hurting for travel expenses, and isn't going to have to go without food or other essentials in order to afford to go to an open house or two. In that case, it surely can't hurt kid to go and get a better feel for the school and let them see that kid is interested enough in them to make an effort. Being that much more memorable to admissions staff / adcoms / etc. may also be beneficial.
If someone lacked the financial resources to go visiting, I don't think that it would substantially hurt them. It isn't like, if an applicant has a good chance of getting in, the school is going to pass them over because they didn't go to an open house. But if they are marginal at all, going might get them a tiny advantage.
This assumes that "helpful" is entirely based on enhancing admission potential. By that definition, I am giving you a weak yes, sure, it could help.
Another meaning of "helpful" would be about kid getting an opportunity to evaluate the culture of the institution. My school has a reputation for being exceedingly strict in ways that many traditional undergrads have not experienced before. Although the academics and outcomes are sound, a lot of people really resent things like the dress code, mandatory lecture attendance (esp. for students who choose the lecture pathway,) and the restriction of food and drink to a designated area rather than being permitted throughout campus. Those are quality of life / cultural issues that really, really matter to some folks, and it would be better for everyone if students who don't want to go to school in that kind of environment would self-select to go to other schools. That is just one example, but each and every school is a good fit for some people and a poor fit for others.
We get so focused on this frantic game of musical chairs and the desperation to find a seat that it is easy to forget that there are some seats that we'd be better off passing over, even if they would be open for the taking. Going to an open house might be extremely helpful if it let kid find out that admission to that school would make for years of misery.