I think precession occurs to a greater or lesser degree depending on the amount of friction caused by worn bearings, weight of rotor, drag from surface finish of the rotor, out of balance etc.
That's not the definition of precession. That's just bad engineering or maintenance.
As for precession, I'm not so sure about that explanation, how does the planet have anyting to do with it if you are not in contact with said planet?
You are flying relative to the surface of the earth, not flying relative to a fixed dot in space. What you call a "straight" track is actually a curve across the surface of the earth - simply because the earth is round not flat. If the earth rotates and you don't rotate your DI with it, your DI will be off after a while. Well, not "off" in the absolute sense, but it will no longer be aligned with the earths grid (or magnetic north). That's precession. (Well, not quite, but close enough.)