In my department at ProdigalU, we have weekly seminars covering the full range of subjects across our department, and we also have topical guest speakers less regularly. Attendance for first year grad students to the weekly seminar is mandatory, but after that, attendance is optional. Attendance at topical seminars is always optional. At first, I was surprised by how few students actually took advantage of the opportunity to see some outstanding speakers at the top of their respective fields, even if they are outside the students' immediate area of research. But then, I realized how few faculty actually attend when the topic is not research relevant, and it all became clear. The students are taking their cues from the professors.
At PhDU, it was in the departmental culture for all faculty to attend the weekly seminar. For more topical seminars, all faculty in that area would attend. As a result, it was the norm for students to attend weekly seminar, and also to attend topical seminars in their areas. I think this is a much better departmental norm for students and for the speakers (who have a large, diverse audience).
A good seminar is organized such that non-experts can follow and find interest in at least the first section of the talk. As a result, I find that I often get ideas when attending seminars outside my immediate area. I also find that such information becomes useful and/or interesting at some point in the future as my research evolves, and then I have a starting point to start out. Furthermore, I find this so helpful, that I sign up to meet seminar speakers as often as I can (though I wait until a day or so before the schedule is set for people outside my area so that my colleagues who have a more direct interest have a chance). Meeting with the seminar speaker is a way to find overlap, meet people in my broad field, and network all without ever leaving ProdigalU.
I do my bit to work on the departmental culture--I come to weekly seminar when I am on campus, and I encourage my students to attend as well. However, my observations of new faculty arriving after me suggest that it is more likely that departmental newbies will adopt the norm (of not attending without direct personal benefit). Maybe I am the only one who sees seminars in my broad field but outside my area as something useful?