I was just thinking about this a couple of days ago. How dumb would it be if, instead of using diagonal size for TVs back in the 1930's they would have used the complete area of the TV? We can even find a funky name for it! A Squinch!
Think about it. Your small 13" TV would actually be a 81 Squinch TV! Wow. Sounds much more impressive than a measly 13". So the, the next step is if somebody told you: "Want to upgrade to a 192 Squinch TV?" you immediatly think to yourself "Man. That rocks. Twice as large!!". But of course, a 192 Inch TV is only a 20" diagonal.. Not as impressive.
A 40" TV would actually equate to a 768 Squinch TV. That's almost ten times as large as our measly 81 Squinch TV. 7.68 Centa-squinch, if you will. Now that really rocks.
Conversely, using a diagonal pixel count (Pixonals!) instead of the impressive Mega-pixel woud make your old digital camera a 288 pixonal camera (that's 1 MegaPixel). The more advanced one would be 500 Pixonal digital camera (that's 3 MegaPixels). And the high-end shooters today would have a CCD which is 816 Pixonal (for those keeping count - 8 MegaPixels). Might be less impressive than using Megapixel, but it gives people a better feel as to what to expect from an upgrade.