There's usually something new to learn (or be reminded of)however small, with most seminars whatever one's level of experience/expertise.
Anyone who has had a previous commercial career where, for example, public speaking is required and has had training for it will, probably, benefit less from a course on that subject.
Similarly, someone experienced in sales and marketing should have a good grasp already of what's required.
On the other hand, anyone with limited or no experience in either area should find training very useful.
I found the Randy Bartlett 1% DVDs to be very good although they did cover a number of common potential mistakes I thought should be obvious.