What do you think of the planar ally suite of spells? Have you thought through the full implications of them as thoroughly as you have the bindings?
As mentioned, there is no PC cleric in my campaign, I haven't given it much thought - but as another poster pointed out, it is the
deity who makes the choice of what to send, so the DM retains more control.
Under Demands of Service, if a spellcaster refused to make any demands for the day I'd still force the charisma check under the basis that choosing 'not to choose' is still a choice.
So would I - I was just pointing out that the spell
could be interpreted another way. Unfortunately,
planar binding has lots of ambiguities.
I've heard rumors (I think on the Andy Collins boards) that the various planar * spells are going to be reworked and clarified in the revised edition -which is good. In one recent session of mine, a cleric NPC called up an arcane angel from one of Monte Cook's Books of Eldritch Might, and the thing was incredibly powerful for a 4th level spell.
I hope they get an overhaul - I would guess that it's a fairly popular spell, and WOTC must have received feedback on it. But I suspect that your experience with the arcane angel is pretty common - supplements (especially 3rd party ones) are full of creatures which can throw the spells out of whack quickly.
(2) Allow the bound creature to use 1/2 its HD, or 1/2 its CR, as a modifier to its escape check. After all, if the caster gets to apply half his caster level, why shouldn't the bound creature get to apply the equivalent?
I used something similar for a long while. I also thought about turning into a Will Saving Throw. The FAQ states
Ability checks are likewise not subject to automatic
failure or success. (No matter how lucky you are, you are just
not going to kick down a castle wall.) If the DM feels that
chance might affect the outcome, a saving throw or attack roll
is probably more appropriate.
I have to admit, it
is kind of nice to have that element of randomness - the best laid plans etc. But I like the charisma check for the raw 'battle of wills' flavour,' and it would be a shame to lose it. If you tinker with the SR as well, I think it can be meaningfully retained.
.... I sure Wish this binding circle wasn't impeding me so ....
You are a very mean DM

I have to say I wouldn't be that nasty - who knows if the Pit Fiend has already used his
wish for the year? But it leads to another question - will a
wish break you out? Would a fiend be cautious of using a
wish this way - he might get an over-literal interpretation.
Another thing that occurred to me and then subsequently forgot to add, is that a
planar binding might be used by, say, a Demon to conjure a player character and confine him in the abyss - in fact, I wouldn't allow this, but I wonder if there has ever been any official word on it.
Another blip that should be addressed is the issue of whether the target is on the same plane currently - if it is, then I rule the spell fails, but the spell description is less than clear. It could also lead to the scenario:
Mage Player: I use
discern location to find out where the balor is.
DM: It's on the Material Plane, in the city of Jeshat.
Mage Player: Damn. I'll have to
plane shift to the elemental plane of air, and use my
planar binding there.
This is how I'd play it, but it seems a bit contrived.