My understanding is that the extra port is free, so Sony figured, why not? (Noob here, all feel free to correct me if I'm wrong). As noted you can configure it any way you want--2 HDTVs, or 1 monitor, 1 LDTV etc. Will many use it? Doubtful. But it's there, it didn't cost anything so who cares? That said, it may very well come in handy if anyone gets creative. I think the general thought is: if you are playing online, on your coffee table, you would have your flat screen computer monitor (almost every geek has at least one of those by now) with IM up and/or a videoconference of your opponent together with all the online menus and whatnot and then, up on the CRT or HD TV, (whatever you are lucky enough to have) would be the primary game action. This setup could be especially handy for a multiplayer version of that eyetoy game with the babes -- Eyedentify. I think it could be most useful for this type of format, and not just for the obvious solution playing GT4 in B spec with the stats on one screen and the road on the other. Picture in picture could be much simpler, but that has its own problems, not least of which is your opponent's big head is going to hog up space on your TV screen.
BTW, does anyone know if you will be able to hook up a QWERTY keyboard through the USB? Would come in handy for turn-based games.
Anyhoo, while it does still sound like a minor pain to set up two monitors, it sounds easy enough to at least *try* it, the issue, which was raised, is whether game designers will take advantage of it or not. Much like the HDD debate, they probably can't assume everyone has two monitors. (Which is why I'm all for including a small HDD, to improve load times, if nothing else).
Anyway, IMHO the dual output set up is an experiment, a cheap one at that. I'm sure Sony/Toshiba would love to sell you two cell based TVs while they are at it too.
Huge selling point? Of course not. But, unlike an HDD, it's a freebie, so it's a nice little touch to pitch as a minor selling point to hardcore gamers that might go through the trouble of a dual monitor set up. Frankly I can't understand what all the hubbub is about, it's almost as dumb as the handwringing about 7 controllers instead of 8. Anyway, if anyone can tell me that the dual output option costs more than $.50 per unit, I'll be happy to change my mind, but as far as I know, it doesn't cost squat.