On Mon, 29 Dec 2008 10:10:21 -0800 (PST), wrote:

I would guess that the main concern is not siphoning action collapsing
the tank, but it taking enough of the water out so that either the
electric elements then burn out

If siphoning would suck enough water out to uncover an electric
heating element, having a vacuum relief valve wouldn't help. It
doesn't just make the vacuum disappear. It gets rid of it by allowing
air to enter.

or steam is generated with a gas unit,
exploding the tank.

I don't know about gas, except the kind we can't discuss politely.

Of course the TPR valve is there to hopefully
prevent the latter.

Here in NJ, I've never seen one used, even in new construction. So,
apparently here they are not required by code. The first time I ever
even saw one on a water heater was on a recent episode of This Old
House, when they were removing a water heater. All in all, unless
I'm missing something, I don't see the need.