In a speaking class I have a student who is extremely shy, doesn't say much and if she does, she gets so excited that she can hardly string a sentence together.
Last week, someone mentioned World of Warcraft in class, which led me to tell them about Second Life. The shy student told me today that she created an account, went to a community I had mentioned, chatted with many people and made several friends from different countries. She was even offered to take on an active role in the community.
Maybe virtual worlds are still not mainstream but there are many students out there for whom this seems to be a good environment to overcome their shyness and practise their language skills. In the virtual world, they can opt in an out whenever they want, which is not so easily possible in the classroom.
I think occasional technical issues are not a deterrent for students who go to virtual worlds to practise their language skills informally as they can always come back at another time. They are more of an issue for (paid) scheduled classes.
So, even if teachers don't have the change to integrate virtual worlds into their lessons and even if they don't feel ready to teach there, we owe it to our students to mention their existence so the students can find out for themselves whether it is a good way for them to improve their language skills or not.