The following question is posed with the assumption that slavery is a bad thing.
Does a magically summoned creature appear because they want to serve, or because they are compelled to serve? If the answer is the latter, then one should always refrain from using such spells.
By similar reasoning, spells that enthrall, command, or otherwise compel a creature should also be avoided.
The phrase, "The ends justify the means," should never be accepted as a reason to use such enslaving spells. If it were to be accepted, then that would open the flood-gates to reasoning that could easily justify any slavery.
The original question of this paper is an actual question, and not rhetorical. As I do not have any such spells myself, I cannot simply summon a creature to ask them why they obey (also, that would be like kidnapping someone just to ask them if they like being kidnapped). If anyone knows of resources that I might use to answer this question, I would be most appreciative if these resources were share with me.