I think this is one of the more important things you can learn. Of all the jobs you can learn about the set, I think this is one that should not be skipped. In most film programs now they include "Acting and Directing for Camera" as part of the curriculum. I haven't taken a class in this yet, but I want to. It just seems enriching!
I've acted in a few shorts, but have never taken any acting classes. I think it'd be a better window into the world of an actor because when you have joint classes like these (where directors work with actors in the same class) everyone is on their "best" behaviour. As those classes are probably the best for us filmmakers, everyone is kind of predisposed to trying to impress each other (i.e. the directors try to impress the actors and the actors trying to impress the director.) I would also love to be able to join an acting seminar (even without the directors part) to see what it's really like for them.
It's fun to read various acting boards to learn what many actors are thinking or to see how they view the director. Many boards talk about how being an actor is much harder than being a director. Other boards have actors that plug us the directors and say something like "Work with this director he knows what he's doing, he was so kind." I would love to learn about all the positive and negative things serious actors think about when it comes to collaborating with a director. I see it and respect it as an art form. These people have spent so many years honing their craft and have decided to make a living out of their talent. They're not to be overlooked.
I think if you can fit these type of classes or seminars in your schedule, you should go for it!