Not exactly rare. And while it does occur sometimes when near storms, really all you need is some particles in the air rubbing up against each other creating electric fields. The times I have seen it were in clouds, not necessarily associated with turbulence and also not necessarily followed by a lightning strike.
Have just finished some study on lightning strikes and found some interesting stuff. Didn't know most strikes on airplanes are negative strikes but some (less than 5%) are positive strikes. The positive ones are the ones that can and often do real damage. Negative strikes leave pits and tiny holes. Positive ones can knock stuff off airplanes.
The thing that most associate with lightning is ignition of the fuel tanks and from what I could find, that is an extremely rare occurrence, especially in jets with the very high flash point of jet fuel. With avgas there is a slightly higher risk.
With St. Elmo's you can take your finger and walk some of the charge around on the outside of the window and yes, it does screw up your comm/nav sometimes.