I'm probably a bit biased because workshops are what I mostly do..
I think workshops can be either a great benefit to the individual, or a total waste of time. I'll submit the following thoughts:
a. Buyer beware. It's the attendee's responsibility to ensure they research the workshop, the instructor, and ensure they're signing up for what they desire/need. Make a list of what you want or hope to accomplish and ask pointed questions.
b. The instructor has a secondary responsibility to ensure the attendee is being properly placed. It hurts everyone to mis-match the client and the workshop. Spend time interviewing the client and be 100% upfront on where they fit in. Offer alternatives.
c. I can see group workshops providing a tour w/friends type experience. Still, objectives should be stated on both sides and matched/accepted.
d. With individual workshops state your needs, ask the instructor to provide a list of objectives to meet your needs, and then 'use' the instructor as you would any other tool. As a client keep your own list and ensure objectives are being met throughout the period of the workshop.
I find every student is different, but many are the same. Each has their own needs and should be evaluated as you go. I never assume they know anything, but in order to not be patronizing I attempt to assess their skills by less direct means if possible.
Many/most clients have no idea what their objectives should be. I spend a lot of time helping them define goals and set objectives and we revisit them throughout the day. I think many would be happy just hanging out and being shown the area and in fact hire me just for this. I try to think long term though because I want them to improve and return for more. I've had those who directly tell me they don't expect to learn a thing, but just want someone to hang out with in the nightlife areas and then offer to pay for all the entertainment/drinks while we do just that. On the other hand I've had journalists hire me just to improve very specific areas in preparation for an upcoming assignment.
It's all about expectations and reality. If you know what you want/need, then research the available workshops to see if you can find it. If you're not sure, contact a few different instructors and ask their their help in determining what you need. The interaction even via email can help you determine if the instructor is someone you get along with, how helpful they are, and if their way of answering questions melds with the way you prefer to learn.
Personally I'm not hot on group workshops. I used to do a lot of them, but when the economy went sour there wasn't much business and I started doing a lot more individual workshops.. and discovered how useful they can be. You make a lot less as an instructor, but its much more satisfying to work with an interested party 1:1 and really be able to help them meet their goals. I'm finding my clients are returning for 3-4-5 different individual workshops because they feel they're getting something out of them. Before, it was rare I'd get a return group workshop client.
And don't be afraid to turn down an instructor, or an instructor a client. Ensure there's a good fit, otherwise it hurts everyone.