I'm considering allowing the hand(s) holding a wizard's bonded object to count as empty for spells with somatic components. Mostly I think this will allow bonded staves or weapons to be a bit more viable, since meta rods can be held in the other hand.
What exploits am I missing that make this a bad idea?
Well, you are missing material components as well.
Probably just easier to say that bonded objects must be on your person, not held in hand.
Sure, its a little goofy that Gandalf can use his staff to power his spells without having it in hand...but I think there are less possible ramifications than if you start changing how other things work.
I can't think of unintended bad consequences of the top of my head, but I think changing the rule that is the problem (holding bonded objects) is what should be changed rather than other ancillary rules that interact with the bad one.
Argh, material components, yep, that's a thing.
The best bonded objects are rings and amulets. Sure, they take up a slot, but you can also enchant them without waiting for the right feat.
When bonded objects are weapons, you have to live with the problem of not having enough hands to do everything you want. No metamagic wands for you.
Choices have consequences. Don't houserule those consequences out of existence.
I'm aware that rings and amulets are the best choices, and that weapons are suboptimal. When one choice, weapons, is enough weaker I don't see a problem houseruling it to bring it closer, but still inferior I would say. Without some change it seems to me that weapons are a non-choice, so some kind of change here actually increaeses player options, which is good. What I want to poke into before making changes is if there is some exploit or unforseen effect that makes weapons the new best choice, rather than just making them viable.
At 5th level, an arcane duelist gains the arcane bond ability as a wizard, using a weapon as his bonded item. He may not choose a familiar or other type of bonded item. He may use the hand holding his bonded weapon for somatic components.
There's some precedent for allowing it, as arcane duelist bards get this... and let us consider that any wizard taking a weapon as a bonded item is already nerfing themselves by investing in a weapon as a wizard.
Choices have consequences, yes, but there's still plenty of consequence even if you override the one specific point in this particular case.
The best bonded objects are rings and amulets. Sure, they take up a slot, but you can also enchant them without waiting for the right feat.
When bonded objects are weapons, you have to live with the problem of not having enough hands to do everything you want. No metamagic wands for you.
Choices have consequences. Don't houserule those consequences out of existence.
It has consequences that are dumb.
There is no downside if you choose a ring or amulet, except that it takes up a slot. A slot that you can then enchant as though you had the correct feat, so no drawback there. Hell, even a solid bonus.
Now, if you choose a staff, weapon, other handheld item you are basically just screwed for trying to choose a flavorful option, because you wanted to be Gandalf.
The problem is choosing any handheld item is a liability compared to just choosing the ring or amulet, which have virtually no downside.
You shouldn't punish people for wanting what is a cosmetic effect. Otherwise everyone just keeps using rings and amulets and there is no diversity.
I think it would be fine if you can use the hand holding the weapon for somatic components. It makes sense to me thematically and it still imposes the flavorful and mechanical consequence of needing the item in your hand.
I'm not a fan of just letting someone have the time on their person. I think it's a massive cop out on the flavor and intended consequence of having a bonded item.