Been one, years ago, and we hire them now as well. They're an essential part of our department and have been for years now because:
1) Tasks we find really tedious, the good ones can wring some learning out of,
2) They're cheaper than us, and
3) We hire and train the most promising ones.
Our new co-ops support larger teams working on larger jobs, where there is a wider diversity of tasks they're capable of doing. We start them off small, with well-defined, well-explained tasks, and then give them more as they learn. The more senior ones, returning for a 2nd term, get assigned to help out on multiple, smaller projects. This gives them more exposure to the breadth of what we do, more people to learn from, etc.
We do try to get them out in the shop, to compare drawings against what's been constructed etc.- it's very valuable experience for them, and good for us too because they sometimes see things that people who have been wearing the project goggles for several months tend to miss- as long as they're not counted on as the sole source of checking!