There is no such thing as too much RAM, at least when RAM is as inexpensive as it is right now. Some applications know how to take advantage of additional RAM and will do so for improved in-app performance. Regardless, you'll be able to run more applications simultaneously and your Mini will automatically set aside a larger RAM cache for the graphics subsystem, which is (generally but not always) better. I have 16gb in my Mini and I'm glad to have it.
I ordered and installed 16GB in my new mini as well.
1. It was cheap ($60 on Amazon).
2. I can give 8GB to virtual machines running Parallels 8 and maximize performance running two OSs simultaneously.
3. Some applications might occasionally benefit from it.
4. It was cheap.
Yes, I usually see 10GB free, but no matter what I do, my "pageouts" are always zero.