Fingerpicking for Jazz, Blues, and Funk
As I have transitioned from playing with a pick to finger style I noticed that I tend to use my right hand thumb (I'm right handed.) on all of the strings, especially the treble ones. I studied classical guitar for about 5 terms at the university and this approach isn't the norm in that world.
The reason I use my thumb so much is simple: The thumb gives the strongest and most dynamic sound. (Wes Montgomery used his thumb exclusively.) While each finger has its own characteristic sound, the thumb is the one that delivers a real punch.