The Planar Binding spells are set up the way they are in an attempt to balance the spell. The balance they are going for, however, is not simply one of a 1 to 1 HD return on whatever you call, but also a matter of balancing the resources that can be gained by calling and binding outsiders.
I don't agree with your attempt at a 'fix', since I don't believe there is anything to fix. The spell is not supposed to operate in a conventional fashion, because it potentially affords the caster quite alot of power, if used correctly. As such, the HD limit exist, not to make some kind of coherent comparison between caster HD and called creature HD, but to limit the special abilities and spell-like abilities of creatures called.
An example: We raise the HD cap to 8. What happens? Everyone summons a Succubus. It's not even a guess, this is going to happen. You raise the cap, people start calling in the sex-demons. And understandably so, because that creature is obscenely powerful at that level. Is it powerful, in the sense that a party of level 10 characters cannot defeat it in fair combat? No. Of course not. But you're not talking about that, you're talking about calling this thing into your service, making a CR7 demon, with abilities stacked to the ceiling, your minion. That is alot of power. An at-will, DC 22, charm-person, rapid-fire machinegun that can teleport, detect thoughts and fire off a suggestion every 6 seconds? The playing field not so much changed, as it got flipped over, and completely redecorated.
Limiting the spell-like abilities and funky nonsense that draws in conjuration-fans like myself, is necessary to maintain a semblance of balance, as you wander into the double digit levels.
my two cents.
-Nearyn
