Dan,
I know what you mean with regards to ptfe i have worked with the stuff in various forms over the past twenty odd years solid, tape, sheet, even a liquid type sealing form, i think they even do a cord now with ptfe impregnated in to it... and like you have also found its failures from time to time.
I have become wise to the various situations that it should be used in and (not).
Julian, I can also echo your thoughts re: the use or non use of loctite as for those asking the questions "why are we even talking about this issue" in respect of the (op) he was merly asking the question should he use it or not and what purpose does it serve.... it had nothing to do with the cost of the drain plug and everything to do with the curiosity of having to use loctite or not and why..
If people don't know, they ask the question, however i still stand my point manually applied loctite or factory cured on the grub screw, unless you degrease the threads to which its being applied it renders the stuff useless.
Dave.