Its bizarre actually who does use the Captain !!!!e.
BALPA do until you tell them not to.
If you have interactions with some Mil units the letters etc are addressed to Capt. but I can understand this with rank structure etc.
I don't mind work stuff to be honest going to the company but if its sent too my home address I get a bit annoyed especially when they just presume even after filling a form with Mr on it. Once ordered some 4 bars from an online shop because I lost mine and they got delivered to Capt MJ.
There are some knob Jockeys out there that use it on every bit of paper work they can get it on. I have no doubt There are cards/paperwork out there with Capt Robert Weaver on them.
I haven't a clue who should use it and who shouldn't, Some areas of the world every pilot is called Captain. The only time I get called Captain in Europe is when very rarely when I have had to have words with handling or ground crew or the ramp rats are taking the piss.