I totally agree that multiple orange ioun stones shouldn't stack.
And I agree with Karui that the extra duration and extra damage isn't really worth 135K either. However, the bonus to spell penetration, concentration checks, and for resisting the effects of dispel magic are all worth probably somewhere in that neighborhood.
I do agree that there are definitely some problems out there, with some spell effects, namely Holy Word, Blasphemy, etc. With a bump in caster level, suddenly your caster level is higher than many of the monster's HD that you will be encountering.
However, clerics can use a strand of prayer beads to add +4 to their caster level for 10 minutes a day, which is plenty of time to put down a ton of buffs for the party especially if you have pearls of power that go with it. The strand isn't anywhere near 4x4x15,000 in cost and it has a number of additional uses.
In any event, I definitely see the possiblily for abuse, but I was curious if anybody had ever seen anything in a FAQ or errata that addressed this.