Aside from the coolness factor of binding an outsider to your will and/or a contract to do your bidding, there's a few things that just don't make sense to me about it.
Primarily, given the level of outsider you are capable of calling with any specific spell being rather limited by that spell, and the availability of that spell being limited by your personal level...what real advantage does calling an outsider have? I mean, how useful is a 6HD at level 10? It doesn't even seem to your advantage until you have the greater versions. (I'm choosing the spontaneous caster levels because they have higher charisma and are more likely to succeed at their respective spells' deals) If it were more powerful than you, I could see going through all the trouble these spells require...gifts, magic circles, dimensional anchors, the chance of a demon busting out and eating your face for the insult of calling him...but something I could probably solo if it came down to it? Seems...iffy. Especially if you have to pay the guy on top of blowing a high level spell just to say hello.
Don't get me wrong, it's really cool...but...
Well, I know I'm probably wrong, and there's applications I'm just not thinking of that make it actually worth all the fuss, but I'll be darned if I can figure out what they are.
You don't need the spontanious caster progression if you take the wizard and diabolist route.
There's an opportunity value (as opposed to cost) to being able to bring in virtually unlimited backup. Plus, many outsiders have summoning abilities themselves. So, it doesn't take long to get ridiculous.
One 6HD outsider might not be that awesome at level 9 or 10 (but free cannon fodder is always nice.)
You can use it to get spells/abilities that you don't know or aren't on you class list (call an imp to cast commune), for instance.
It has a long duration, so you can cast it during downtime to accumulate allies: a 10th level sorcerer with 20 Chr and Spell Focus: Conjuration can accumulate about three Barbed Devils or Hound Archons per day. With a 10-day duration, that's a lot of disposable minions. (In exchange for three spells known and most 3rd level or higher spell slots.)
You don't need the spontanious caster progression if you take the wizard and diabolist route.
Diabolist requires casting a sixth level spell to get in. Some of us don't have GMs that let us get our hands on scrolls way above our level on demand.
Also, that's only useful for devils, and it's pidgeonholing your entire build around one spell. Doesn't help explain why an average caster not specialising in it would cast it.
You don't need the spontanious caster progression if you take the wizard and diabolist route.Diabolist requires casting a sixth level spell to get in. Some of us don't have GMs that let us get our hands on scrolls way above our level on demand.
Also, that's only useful for devils, and it's pidgeonholing your entire build around one spell. Doesn't help explain why an average caster not specialising in it would cast it.
Lesser planar binding does work for getting into Diabolist, but requiring a fifth level spell isn't much better if your GM doesn't make it easy to get higher-level scrolls (or if you feel weird about your character buying a scroll to call a lemure for purely mechanical reasons.)
Anyway, there's very little reason for a Sorcerer to cast it if they don't plan on focusing on it: it costs a lot of Spells Known to really work, so there's no reason to invest unless you plan on really using it.
For a wizard, it can be a handy spell to have in a pinch, because like the summon monster spells, it has a lot of versatility. You probably won't prepare it every day, but it's a really good 'downtime' spell (fireball doesn't do much in peacetime.)
For instance, you can call a zelekuht inevitable to track down a recurring foe who keeps escaping. You can call squads of outsiders as flanking partners, cannon fodder, and meat shields. You can access all kinds of non sor/wiz spells via outsiders' SLAs.
You don't need the spontanious caster progression if you take the wizard and diabolist route.Diabolist requires casting a sixth level spell to get in. Some of us don't have GMs that let us get our hands on scrolls way above our level on demand.
Also, that's only useful for devils, and it's pidgeonholing your entire build around one spell. Doesn't help explain why an average caster not specialising in it would cast it.
In addition to what Ross wrote, clerics with the Void domain get Lesser Planar Binding as a 4th level spell. Along with the other binding spells and the human favoured class bonus of spell penetration, this makes them probably the best diabolist around. So good at it, that they might not need the prestige class at all, thus opening up every alignment and therefore outsider.