For me, it is hard not to justify putting APRS in my -9 when the cost is so low (around $200) and the benefits are so great.
1) It is just some cool technolgy and neat to see your tracks
2) It gives a measure of security to those in your family who are non-aviation minded.
I showed my wife and kids how to use it at home.
3) It should be a great benefit to SAR combined with a PLB in the event I need to be plucked out of the wilderness somewhere.
4) It is educational and makes me feel like I am on the "cutting edge" of technology.