According to HEA, this is always on. I don't think it should be a system preference. Libraries that *really* don't want this useful feature (which don't seem to exist at the moment) could still hide it with css.
It relies heavily on JavaScript and doesn't work right without. I'd like to keep the option to deactivate it in a clean way.
Well the decision to turn the syspref on or off belongs to the library, not to the end user that has javascript or not. So hypothetical users that don't have javascript will see an interface that they cannot use indeed, and won't have their say about it anyway. I would argue such clients probably deal with this somehow already considering how common it is. If this is really an issue, then this feature should be refactored to work without javascript. But in all cases i think the system preference here isn't really required, so different issue in my opinion.
I agree that we could remove the system preference after some refactoring. We should also make sure some id and classes are in place to still be able to hide it.
See also Bug 20898.
We have a lot of switches for OPAC features - I am not sure if this pref hurts anyone - but would be nice to get more opinions.