It seems your post is essentially "anti-marketing". Well, let's be honest... marketing is rarely correct. ATI does not make the best GPUs around, regardless of what their frequent NewEgg ads and their 9x00 series cards have told us in days and years past. George Bush is not a good President, regardless of what Republicans will tell you, either... oh, and **** Cheney's got terrible aim. All sarcasm aside, marketing is made to be bull****. However, I think you might very well be missing the focus of SLI.
SLI is essentially made as a stop-gap solution. Say I'm building a 939 system, and I don't have $500 to spend on a 7800GTX (which I'd love) or $750 for a 512MB version of the card. Now, I have two options... the first is spend $60 on a 6200TC and have it use 192MB of my system memory to supplement it's existing 64MB. I then have a 256MB video card, and a really ****ty one at that. Or, since I'm already building a system, I can instead chose to, say, cough up $180 on a 6800GS or $290 on a 7800GT, and have great 3D performance. Then, six monthes or a year from now, I can poney up another $150-$250 (probably what those two cards would cost, respectively, after time has taken it's toll) and essentially double my 3D performance.
Not only will this save me money up-front, and allow me to, say, spend more up-front on the CPU or the motherboard, but it'll save me money in the long run by being able to not have to buy two $500 video cards, but instead two $200-$300 video cards.
Now, with this being said, sure I can spend $500 on a 7800GTX initially, and then add a second in a year or so for maybe $350 or $400, and have the same performance-doubling effect. It's kind've like buying a ghetto Kia now, and in six monthes adding enough machine to that Kia to make it a shiny new Nissan, or whatever your preference is. The essential truth (thank you, James Frey) is that you're really not spending that much money, because in the end the idea of SLI actually lets you prolong the amount of time and money you need to spend on upgrading your system.
Now granted, there are some people who went and spent $1500 on two 512MB XFX 7800 GTXs, but you know what I've learned? I've learned that a) I wish I were them, and b) when I am out of college and make $70,000 a year... I WILL be them.
This all depends on what you can allow yourself to justify, and in the past I personally never would've spent $500 on a video card. Nowadays, however, the 7800GTX 256 or 512 MB variation is such a large performance boost that, if I were in the market for a mid-to-high end system, I would consider myself pretty dumb not to invest in one. With that being said, in half a year, I can spend some more cash and let my semi-aging card have new life for an extra year or whatever your preffered duration is.
I guess the point is that SLI is mostly used for prolonging a card/system's life, and very infrequently to try and somehow substitute a higher-end card with two lower-end cards... at least, not out of the box.