This always seemed odd to me: all but the weakest fiends, at least among the "basic three" types (i.e daemons, demons, and devils) can summon others of their kind, but no celestials can (not even Empyreal Lords). Why is this? It's not like summoning is considered an inherently evil power (indeed, using summon monster to summon a good monster is technically a good act), so I'm confused.
No idea, but it seems to be a rather old convention, certainly predating Pathfinder.
My theory is that celestials hate being on the material plane. From their viewpoint we're from a lower realm; it's equivalent to a human being summoned into hell.
I think it's about the armies of Good relying on quality, Evil on numbers.
If I had to guess, it has to do with the mindset of evil outsiders vs. good outsiders. Evil outsiders summon "minions" to act as "spell-/sword-fodder" and/or lackeys; it's not explicitly stated, but the creatures summoned are probably ones that the summoning outsider has some sort of control over (rather than a random outsider of that type that has no connection to the summoner). Good outsiders rely on more consensual relationships; if communication is possible, they would ask for help, rather than compelling help; also, they are more likely to work with local (mortal) groups, where feasible, if they need supporters and can't gain support from the celestial planes.