After my own experience and close analyzing Joe Satriani's playing, the guy who is my greatest influence, I've set up a rough rule for pinky use:
1. From frets 1-12, top three string (high E, B, G), If I'm using my ring finger, I don't use my pinky. Ring finger takes pinky's place, middle finger takes ring finger's place. HOWEVER!
I still practice
using it in these situations.
I have read that your pinky and ring finger share a tendon. This is why it can feel so unnatural to hammer-on from your ring finger to your pinky. The pinky is also, IMO, the least "toneful" of fingers because it lacks the strength and mass of your other fingers. Things can actually sound better if you don't use it.
2. Frets 1-12, bottom three strings; use your pinky and ring fingers.
Using your middle and ring fingers together here can be akward. This is where your pinky becomes neccesay and all that practice (hint hint) you did in rule # 1 comes into play. The strings are thicker and therefore make up for the lack of pinky mass.
3. Above the 12th fret...all bets are off.
Above the 12th fret, anything goes. You can now stretch between two, three, or more positions on the fretboard. Why would you NOT want to use your pinky?