You could house-rule that of course but I was talking RAW. In 4E an action is either a free action or it's not. There aren't different kinds or types of free actions.
Nor was a quickened spell a different type of free action. It was just a free action you could only do once per turn.
IMO saying that you could only change grips once but allowing multiple other free actions doesn't make a lot of logical sense.
Then your logic fu may be weak. How much does singing interfere with your ability to juggle? How about your ability to tap a rhythm with your foot? Each of these things are a bit separated so that's something, at least, though in truth there is _some_ impact to quality but you can still do these things.
Now, while singing, juggling, and tapping a rhythm with your feet, recite a spell, parry attacks with your magic shield hand, and dodge back and forth to assist your own attacks. Oh, and launch the juggled balls at enemies bouncing them back to your hand.
Okay, getting a bit more complicated.
Switching grips takes as long as it takes to say "four". Switching back takes as long as it takes to say "score".
Potentially, but gripping something, leveraging that grip, taking a swing, then releasing the grip, takes more than "four score".
So where's the line?
Thankfully, the RAW answers that:
"The DM can restrict the number of free actions in a turn." and it even gives an example of a limited free action. "Speaking a few sentences" - you can only do that once, you can't do it twenty times
Unless you are going to rule that you can't say "four score" in one round, you are doing it JUST to stop swordmages from getting a +1 damage NOT because it makes any sense.
Saying "four score" makes sense. Saying the whole Gettysburg address doesn't. So there's your line of sense. Similarly, I don't think it "makes sense" for them to design the swordmage so that it has a restriction unless it has a restriction, nor do I think that it makes sense to allow grip swapping an infinite number of times if you're using that other hand to gain AC.
Thankfully, I don't have to care how it works, but as I said before - every table I've been to, it's been ruled the same. You pick one grip for the turn, take it as a free action. So you pick between +1 damage or +2 AC.
Since it doesn't make logical sense to limit it to once per round
As noted, your logic fu is weak. It definitely makes sense to limit it to _some number_ per round. I'd imagine no one thinks you can change grips one hundred times per round. I already know some people think one time per round. So each DM will pick their own number from there. We've already established there _is a limit_, we're just haggling.
the only reason left for that house-rule
Except it's not a house rule
is that the +1 damage is overpowering somehow. That I just don't see. Many other things in 4E are stronger and/or cheesier than the swordmage grip change that are perfectly RAW and legal.
It's no more overpowered than giving any character +1 damage. So let me know when you decide to give rogues +1 damage, or fighters, or warlords, or clerics, etc.
