I had to face a similar problem while playing a LG sorcerer. I had summon monstered a bunch of Bralani celestials, and I sent them to a certain death against a creature they could hardly annoy (think sentient sphere of annihilation) in order to slow it down and allow for a friend to escape. Actually, I wasn't even sure that they would have slowed it down, and the guy was likely to be fine anyway. Suicidal order for very little benefit, basically.
This got me a couple of comments on my alignment, to which I answered: "They are summoned creatures, not conjured ones. They cannot die, they only go back to their plane. And they have been disintegrated, so they didn't even suffer. I didn't enjoy doing it, but I certainly don't feel guilty about it. I will send to certain death a million summoned celestials before risking the loss of a friend". That character really doesn't like taking chances.
Sticking to the PHB, summoned creatures are physically and really there, but instead of dying they return home. This means that they feel pain, but that's it. IMO, even a good character should feel justified in giving them suicidal orders, except in very strange circumstances. Putting them through pain for nothing is certainly evil, but as long as there is a good reason there should be no problem.
What do they think about it? Intelligent outsiders probably know about the possibility of being summoned, and know that they aren't really risking anything permanent. Celestials will be fine with getting "killed" or even suffering for a good cause. As paragons of good, they are ready to sacrifice their real lives for Good, so what's a bit of pain? I expect that they need very little justification. They will never like being used for selfish or evil purposes, but again I think that to them, pain and a virtual "death" are secondary to the purpose.
Fiends will not be so enthusiast; being extremely egoist they'll not want to suffer, no matter the reason. Maybe, just maybe, a Lawful fiend will accept that he has to suffer to further the plans his master. But who cares what a summoned fiend wants anyway? Whatever you do to him, he probably deserves it. Good characters won't enjoy putting them through needless pain, but they won't feel guilty either.
Low-intelligence outsiders, OTOH, are likely to have no idea of what's happening to them. They will be in total panic. They will be convinced that they are going to die. But they won't.
I think that there is no moral problem in using summoned creatures to do whatever you want. You may have a problem the moment you start to put them through pain or humiliation without a shade of necessity (more so for terrorized creatures that don't know they have nothing to fear, less so for fiends that deserve everything they get). You definitely have a problem if you enjoy doing this. But no good character would find himself in such a situation anyway.