I consider it offensive and the fastest way to get a Bad Reputation.
The way I see it... unless you are Japanese (which is a culture thing of disbanding without notice or regards for the other 5 people in party) OR the party you are SUCKS big time OR you have a RL issue, please stay as long you can.
I am most times Leader when i make XP parties, and only ask people 20 minutes so I can get their rep.
Some people are AWESOME at their jobs and others think because they are NIN PLD BRD COR (they get that ELITIST / irreplaceable air) they can come and leave parties just like ditching a Mickey D's to go play at Burger King...
I will remember you, and many I have seen Lfg a BLU SCH BLM MNK DRK (who used to be the same COR BRD PLD NIN etc) out there won't get any invites from me (or any other pt leaders they ditched) in the past.