Outside of making 3D printers accessible I'm not sure how libraries could feasibly offer workshops. People don't only work in plastic, and presently 3D printing is a novelty for your average person.
With the Chicago library's lab, they offer various workshops on some small example projects (e.g. a 3D printed trinket or a laser cut greeting card) to expose people to the basic process and offer open lab hours. You can look at the schedule here. I'm not sure where you see a problem with the feasibility of this.
3D printing is to a large extent still in the novelty phase, but as it gains in popularity so will the practical applications. I have a broken plastic component of a relay in a 70s era pinball machine for which purchasing a replacement isn't an option. It is however a simple geometric design and I plan on printing a replacement at the library.
Recent investments will yield a slight profit.