Almost always while hiking and I pass someone, one of us will ask about the trail ahead, or distance to the cars (depending on the direction we're traveling). The friendly chit-chat is one reason I love to go backpacking so much. In essence, I think we're not "warning" them, but just giving them an update on conditions, and letting that person decide. I'd never sit there and critique their equipment, since who knows what they have in their pack?