The philosophic part of me tends to agree and would go as far as saying that since so much learning is incidental we shouldn't even limit the computers to scholarly pursuits. If library computers are supposed to substitute as home computers for those who don't have one, why should librarians object to instant messaging or shopping or social sites? I also think that librarians should try and foster happy relationships between patrons and technology, and users are happier when the technology does what they want it to.
On the other hand, I had an interesting talk with a former computer lab supervisor recently, who recounted various unpleasant experiences when trying to enforce the college's reasonable use policy. (The alternate title to this post is one of the reactions he reported to me). Using content blocking software avoids this type of direct confrontation with students.
So, on one side there's the content blocker which allows librarians to worry about other things besides what trouble students are getting into online, and on the other side librarians can strap on the riot gear and personally enforce whatever they interpret 'appropriate use' to mean.
The second way is harder, obviously. And less fun for the librarian. But as much as I hate to say it, I'm slightly more in favor of taking the blame personally than allowing a program to control patron habits. I guess it's just one more item to add to the ever-changing job description...