"BTW, Petrosian, please explain why you think that allowing custom skill items up to say +10 would be a problem due to the skills being of unequal import?"
It might not be, but it could be.
As with ALL cstom items, a Gm who follows the guidelines as if they were rules without a vet on the price is going to find trouble.
If he assumes all skills are of the same worth, then he will have a fairly easy chance to run into trouble as in most cases some skills have a bigger impact than others.
The ring of stealth +10 move silent is probably going to be worth more than the ring of knowledge nobility and both are probably going to be of less worth than the ring of Use magic device. (These values will be subject to the campaign... in some game the ring of knowledge nobility might be the mot powerful.)
The balance between those items will stem from the circumstances the Gm provides that need them.
If he is not careful, as with ANY custom item, problems can occur.
BTW... i don't have such a frothing problem with the "+30 limit" for one simple reason... since the creation of ANY custom items has to go thru me, i can simply say "no."
If they had published a long list of official +30 rings in the DMG erratta, I would have a problem with that.
All +30 max means is you can, IF you think its OK, allow them into your game. (Which makes it frankly a waste of text. I really do not need them limiting my custom items.)