First, I would say it is a common trend to use the 9-Box tool, and not a fad (although of course time will tell!). But here are other ideas about the 9-Box:
It is definitely not a required tool for talent benchstrength (talent review meetings and succession) activities and meetings. I've facilitated very successful meetings without a 9-Box. However, I do think it is a good tool to help with discussions, especially with leaders who are participating in their first Talent Review meeting.
I don't recommend trying to make it a 12 Box or a 15 Box (I've even seen a 24-Box tool). This makes it way too complicated. We will be discussing and practicing using this tool in my next Talent Review-Succession Workshop, if you can join us on Feb. 24-25 in Houston, Texas - you can find the information here: http://conta.cc/1P6OBaW
The 9-Box is a "discussion tool" and it can be a preparation tool for a Talent Review meeting. But it is not a validated talent selection tool, it is not a scientific tool and it is not a decision-maker - the leaders are the decision makers. The 9-Box just helps facilitate talent review and development need discussions. Because it is not a scientific tool, so I don't at all recommend trying to do a forced ranking or a bell curve on the chart.
The 9-Box is more usefully in the "early years" of a company's Talent Review-Succession strategy as the leaders are learning how to discuss and develop talent together as a group. As the years go by, it may not be needed, so from that aspect, it might be seen as a fad over time, but for right now I do think it is helpful.