I have looked at the NASAD standards (and if I remember correctly 5-6 hours contact hours are necessary for 3 credit hour courses for accreditation), but this small institution has not yet pursued NASAD accreditation (right now we only offer a BA). As we are a growing department, I will pursue accreditation in the future, but I am trying to take on one headache at a time, and getting us as close as possible to the requirements before beginning that dialogue with the dean.
I'd just use NASAD standards as the rationale--this is the industry standard, and even if you're not going to pursue accreditation, you want to adhere to an agreed-upon "best practice."
Everyplace I've been, studio courses meet extra time; I've always assumed this was for the reasons you articulate (as much as anything, student artists need supervision in the studio, which is a different thing from reading the book...).
I looked through our paperwork, and I can't find anything about this issue, and our art courses meet 5 hours/week for 3 hours' credit. I gather we've just always done it this way. Surely, though, the NASAD standards have some rationale?