And therein lies the rub. The question isn't whether you can alleviate the issue by changing the grip, but whether this is reasonable.
In my 41 years I've used something like a hundred or so phones regularly, from old-school pulse dial Ericophones in the 70's via mobile NMT bricks in the early 90's to the iPhone 3G. Try as I might, I can't think of one that came with restrictions on where to touch it. I don't think anyone is asking for a phone that works literally no matter how you hold it (upside down, to the back of your head, under water, in your mouth etc), but I think they have the right to expect that as long as you stay within the realm of reasonable ways to hold a phone (right hand, left hand, whatever) there should be no manner of holding it that results in dropped calls.
Most importantly, I think the heart of the issue is that an end user shouldn't have to know how a cellphone works from a technical standpoint. We're all tech geeks here on MR and we talk about antennas and attenuation like it was common knowledge, but this is supposed to be a phone for everyone, and you know perfectly well how clueless most people are about these things and how you need to babysit them through life (McDonalds coffee burn lawsuit, anyone?) The average iPhone user wouldn't even know where to look for the cause of the dropped calls. They'd try stuff like pacing around the house and standing in windows to get good signal, heck, they'd try voodoo before it would occur to them that it's their damn hand killing the signal (talk about last place anyone would look). This is a design quirk you can't track down intuitively without at least some rudimentary knowledge of cellphone technology and electricity. You can't leave stuff like that up to soccer moms, schoolgirls and grandpas to figure out through trial and error, but if you insist on doing it anyway, then you should write it on a big red effing all-caps sticker attached to the phone, and THAT is Apple's real failure here. That's even worse than the design quirk itself. They didn't inform anyone about it, they tried to sneak it under the radar. This issue wouldn't have gained half the traction it has if Apple had been upfront about it from the get go, but they weren't and that's the main reason why people are upset. The reception issue is bad, but the deception issue is the true reason why it snowballed.