Some players refuse/ignore invitations as you describe while others join with a caveat that they may need to bail at any moment. I find (from the party perspective) I much prefer the latter as abrupt disconnects can occur for any number of reasons so I'm not bothered by them (just ask Carson
).
In my experience you and those others who send explanatory remarks are in a courteous minority, i.e. most declinations are without comment and until the options are [
accept|
ignore|
refuse|
graciously decline] seem to come down on the wrong side of etiquette.