This is a smart move...when undertaken carefully. Nothing is wrong with pushing out an interview until more firms come to call. The risk here is that no one knows when all the firms are going to come a-calling. Thus, pushing a meeting out further than two weeks can be dangerous.
Firms, especially Big Law, interview hundreds of candidates, and you must always keep in mind your competition. If a firm wants to see candidates for a given position during a particular week and you ask to schedule your interview two to three weeks later, they will likely comply. However, it is very likely that you won't actually have that interview because the spot will fill beforehand and/or they will already commit to or click with someone else beforehand...and sometimes cancel the meeting altogether.
Very recently, a candidate of mine was offered an interview with a top NYC firm. Although in town on business, he asked that I not schedule any meetings with the firm because he wasn't ready to interview yet; he wanted to get in the mode and get a few more firms lined up before he was ready. So he asked me to move the meeting to three weeks later. While this sounds reasonable and I did ask to reschedule (although I knew the risk and told him about it), the firm has yet to do so.
In effect, I don't know that we will ever get our chance with this firm again. He had his chance and lost it, and the firm has moved on. That was a tough lesson to learn about saturated and highly competitive markets such as New York City.