I am not sure what the REQUIREMENTS are but from what I have been told, there are a couple of things to consider.
There are elements that are in fact important outside of class- like going to their office. This is good.
However, the perspective is best when they see you in more than one setting. How do you interact with your peers in class? Do you show up on time? Do you ask good questions? Are you prepared for class?
It is difficult to make those determinations while only seeing you in office. You must consider what the ADCOM is trying to determine:
Are you someone who really wants to learn?-- Office visits could help determine that
Do you work well in teams?-- not very easy to see personally.
Are you an interdependent learner? -- difficult to say.
etc...
In other words, if there is a way you could take THEIR class (another course) or just cultivate a good relationship with another professor before application time, it is likely better.