Kmart Kommando said:
cost of +5, Bane weapon = 72,000gp
cost of +1, Bane weapon and a Pearl of Power III for your wizard buddy = 17,000gp
ECL system assumes you have X amount of gold at each level in swag.
effective amount of gold you have = X + 55,000gp
That's not a fair comparison. Firstly, you need to have a 20th level wizard buddy in the first place (which isn't exactly low-powered requirement there), and secondly, a pearl of power doesn't free up a slot, it just allows the reuse of that slot (so sure, he could cast GMW again, but that's not the same thing as casting any third level spell).
So yes, a party that has a 20th level wizard in tow willing to spend a (mere 3rd level) spell-slot has an advantage over one that doesn't have such a wizard. That's not a symptom of a broken rule, however.
You're quite right in saying that the rules are argued over and that rules can be interpreted multiple ways. However, D&D provides a standard framework for determining stacking. This framework only considers the type of the bonus, and its source. Bane weapon's description strongly indicates that it's an
improvement to normal stacking. It
increases the pre-existing (i.e.
normal) enhancement bonus. There's no reason I can fathom for supposing that the "normal enhancement bonus" should be anything other than the definition found in the PHB, DMG, and SRD. As a matter of fact, the description stresses that it's just a plain
normal enhancement bonus.
Which is exactly what greater magic weapon provides. Ergo, bane improves the enhancement bonus provided by greater magic weapon.
It's a minor issue that higher-enhancement weapons, which are expensive, aren't particularly useful sometimes. It's not a major balance issue. You can mitigate it's import by using augment crystals and dispel magic. And it's only an issue once 3rd level spell slots are not longer a limiting factor. Sorcerers who want to use this buff need to spend a spell known. Notably, this is a spell that's most useful for other party members - so it promotes party cohesion (a fine thing). It's somewhat unfortunate, the way enhancement bonuses and other weapon enchantments interact in terms of pricing, (such that adding enhancement bonuses causes other enchantments to rise in price), but that's really it. By not letting bane stack, you're really not fixing that problem, since you can still have a +1 flaming, frost, shocking, keen, vicious, brilliant energy weapon - and let the wizard pimp
that to +5, which really isn't any less powerful that the bane issue.
So not allowing bane to improve the enhancement bonus granted by GMW doesn't fix the real issue. More generally, the ease with which an enhancement bonus can be added may be unfortunate, but it's not game-breaking. Finally, there's no reason to assume that the enhancement bonus provided by GMW is somehow abnormal.
Conclusion: let bane+GMW stack, already!