There is no opposition to the EXISTENCE of lucid dreaming. It is surely proven.

The only "opposition" to it is...
As seeker said, association with new age practices or drug use
psychological damages as a result to it (for example, things like dieing in your sleep or not getting enough rest, etc.) These are all false...
Some think it is "occult." For example, the old hag (search it on wikipedia) that you get from sleep paralysis is often seen as possession by a devil (this used to stem people to begin to "exorcise" others).

Most arguments against lucid dreaming stem from ignorance about it.