I am always happy to be 'pax'. It is far easier than the three syllable word for which it substitutes.
When I first learnt of 'slf' I thought it very funny and still do, I think of it in the same breath as referring to an experienced pilot merely as a 'driver'.
My thanks to qfcabin for concern in this but agree with most of the above by saying, it is the service on the phone, at the check-in, at the cabin door, at the seat by which I judge.
In my spare time, I am involved in charity work with recently bereaved people (not religious/church orientated) and we do have some words for them .... it is a way to off set the difficulties that we encounter. Just as it does the folk in the cabin!