Bryan R. Tyler said:
Like joeca0304, I have to completely disagree. Having a number of set plucking patterns is extremely benefical- the plucking hand is just as important in playing as the fretting one. "When you're too concentrated on which finger to use, you're probably not concentrating on the music and groove" could be said about the fretting hand as well- the point is to learn your techniques and how you'll use them so they will become second nature, and you won't have to concentrate to be able to use them. That's what practice is for.
"What ever finger is there and usable, use it." isn't a good way to go about playing. If the song calls for a sharp pick-like attack and you're thudding away with your thumb because it's there, then there's going to be a problem. If the song calls for a galloping triplet feel, using a 3-2-1 or 1-2-3 patten is going to get it better than using just your index finger. And with more complex basslines are called for, you're going to need a set plucking pattern is essential just to play the music right. If you're trying to cover a fast Dominique DiPiazza solo and you're just playing 1-2-1-2, it's not going to sound good and clean, and you will end up exerting way too much energy to try to play it that way. Gary Willis' plucking technique aids greatly in string skipping, muting, and conservation of energy, but it is rather complex and you need to practice at it to get it right (too complex for me!). Most of the pros with the best grooves in the business have a very trained right hand and know exactly what it's doing, and don't have a problem with grooving.
Steve Bailey said that one advantage of using a 3-2-1-2-3 patter over using a 3-2-1-3-2-1 pattern is a more even feel when playing even groups of notes (2, 4, 8, etc.), and to use a 3-2-1 pattern when playing triplet grooves. One advantage I've seen for using a 3-2-1-2-3 over a repeated 3-2-1 is that after plucking with the index or ring fingers, the middle is the next closest to the string, and therefore can pluck it quicker than the finger next to it.
As to the original post, 3-2-1 seems to be a much more instictual feel than 1-2-3, although I'm practicing getting my fingers to play in that order (thumb-1-2 thumb-1-2). I'm also practicing thumb-1-2-3 thumb-1-2-3, although without as much success so far.