I am personally not a big fan of seminars ever. If you are a total beginner and you are looking for an art that fits you, then they might be useful to give you a brief sampling.
There are a couple of problems with seminars, in my opinion.
First, if you are being taught something that is part of your system, that you already study with a good instructor, then why would you need to learn it in a seminar? You should be able to get it from your instructor, who is then in a better position to help you continue developing the skills in the long run.
Second, if you are learning something in a seminar that is not part of the system you already train, then once the seminar is over, you have nobody to work with who can correct your technique as you continue to practice. A seminar is short, maybe lasting anywhere from a couple hours to a weekend to a week. Unless you are very gifted and already experienced in a particular art, what you learn in this short period of time will probably be poorly learned. Then the seminar is over, the teacher is gone, and you are completely on your own to practice it, and since you didn't have enough time to really learn it well, you probably continue to practice it poorly. If this material is also new for your regular teacher, then he cannot really help you with it either because he is struggling as much as you.
Example: lets say you are a karate guy. But your teacher wants to expand the background of your group. So he brings in a teacher in tai chi. But nobody in your group, including your teacher, has any experience in tai chi. So this tai chi teacher comes in for a weekend, and teaches a short version of a form. That's all you would have time for, without prior background. And in that amount of time, you just barely got thru it. You are struggling just to remember the form, and your rendition is lousy because tai chi is a supple and subtle art and it takes a long time of practice under a skilled eye to get it right. But now the teacher is gone, your group stumbles thru the form for a while, and probably eventually drop it along the wayside since deep in your hearts you all know you are doing it wrong and you have no way to correct it. If you want to add tai chi to your background, you need to study under a good teacher, and commit to learning the art as completely as possible, and give it as much time and effort as it takes. A weekend seminar does not do it at all.
Sometimes seminars can be interesting because you get to see a different perspective on your own art. If the guest instructor is teaching an aspect of the art you already study, you might find his approach to be refreshing, and it helps stimulate your drive to train. That can be useful. But after a few of these, sometimes you find that you are less impressed by many of the people giving seminars...
So they CAN be useful under the right circumstances, but I personally am not a big fan of them for the above reasons.