Plastic's a lot cheaper than ss (& a lot cheaper than chromed steel) in size/shape that fingers can easily deal with in tight spaces. The plastic knobs will still pull the metal studs out of the wood if over-tightened.
The length of the metal studs is required, imo, so that there's sufficient stud protruding thru to find the them by feel and still get fingers around the knobs while tightening/loosening them.. .therefore knob design is such that the larger OD (where fingers grasp the knob) is offset from base (where the compression force is applied to the body panel).
From engineering point of view another reason for stud length is that using plastic knobs the plastic threads require more threads engaged (than metal threads) for sufficient friction and load to secure wood & prevent them backing off (with repeated heating/cooling, vibration of the studs).
From my recollection in removing/re-installing my wood twice now (or maybe three times?), the instruments (& glove box, radio) are out when I'm removing or reinstalling the wood. I can't recall though if my side-fresh air vents are also out when I removed/re-installed the wood. I have to remove my instruments again to have glass cleaned and replace either the speedo or tach cable (I forget which is making noise --- I rigged one of them the last time I had them out)... and should have replace it at the time, along with getting my instrument glass cleaned (on the inside). Whenever I remove the instruments I check whether I should attend to the wood finish... and also check the plastic knobs "tightness". Thus far, the plastic knobs have never been "loose" whenever I've had the instruments out.
