As always, the answer is "it depends." It's easy to wind up wasting time on some seminar about a topic too simplistic or too foreign to your current work. For instance, a seminar aimed at learning to program in Fortran may be a waste of your time if you are a web developer and really want to continue that route.
This can be a problem since you only have a fixed number of hours in a day. The time could be better spent reading books/blogs on more relevant technology.
That being said, I believe that relevant, high-quality Webinars/Seminars are quite valuable. I've gained a lot from watching some on S.O.L.I.D. that were posted online a few months ago. Seeing others in the same community tackle similar problems I have faced really opened my eyes to new and better solutions.
Essentially, it depends on the specific webinar/seminar in question. I've watched some that were totally wastes of time, and I've seen others which were very enlightening.