This thread took an interesting turn!

Lucas was influenced by classic tales where supporting characters often don't have the freedoms the main hero might enjoy. The droids and their relationship with each other is thought to have been inspired by a bickering pair of servants in a Kurosawa film.

In the OT era, where aliens are second class citizens under the Empire, I don't think droid rights is as much of an issue with the populace. The cantina scene even shows a bit of droid bigotry. "We don't serve their kind here!" Whether this was meant to tie in at the time to memories of droid armies in the prequels is anyone's guess.

Luke, possibly because of growing up on a desolate rock with mostly jerks for friends, may have more affinity for his mechanical servants. I think there's a passage in the novel where he hesitates before deciding not to put Threepio's restraining bolt back on, after repairing him in Ben's home. At the same time, he isn't hesitant to put the droids in harm's way in Jedi.

I have a gut feeling that when human clones happen in real life, (sure it's outlawed, but somebody is going to do it anyway) or robots achieve a degree of sentience, many of our old prejudices will be transferred to them. After all, you don't want your daughter to marry one. ;)