Then you throw in different picks and pick right at the bridge vs over the middle etc.You can get a lot done with just one finger. I do like using my index finger like a pick. But its hard to keep the tone even flicking it back and forth. . Truth be known though..... a pick is a lot better sounding, faster and it doesn't wear my finger out. But you do have to work at it some. When I practice I'll be sure to do it both ways. Sometimes I'll devote a day to just using one or the other. Sure, initially the finger will rule. But once you get the hang of it. The pick is faster. Now its not faster than multiple fingers. Thats another discussion. Ideally you'll train yourself so that you're indifferent as to whether you use a pick or not.
I try to use index, middle and ring fingers at times as well as thumb but mostly pick.
My 64 year old hands cannot use only an index finger for long and thats okay...i have Micks picks made of stiff synthetic felt that i sand the edges down i so can get close to an index finger sound.