Because the cheap second card would have to be very cheap. A Dual Head2Go is £158, which may be more expensive than a cheap graphics card, but if I did buy a cheap graphics card then I'd have to swap it if I upgraded my PC. As long as I use monitors it supports, I could use a Dual Head2Go for a lot longer.
Also a second graphics card would increase the heat inside my PC, which is already higher than I'd like, it would increase the current draw (the Head2Go would too, but it's 5V at 500mA (2.5W) at the most being USB powered as it is) and increase the noise.
I think my two choices are: Buy a Dual Head2Go if I can get it to drive the surround monitors only, if not I'll sell my 580 and buy a 680 instead. While the H2G puts less stress on the card in triple monitor play, I personally would only be using three monitors whilst my PC is next to my sim cockpit, the rest of the time I'd have it by my desk and I'd only be using it with a single screen, where the 680 would perform better whilst using less power and therefore it'll also be running at a lower temperature. As an added bonus I could use whichever monitors I'd like, which I think will be important as I want to change these 1680x1050 monitors as it is. Having to buy a third to keep a Triple Head2Go happy would be a pain as then I'd have to upgrade all three at once.
I don't mean to hijack the thread or anything, as I'm in the same position as the OP, he might like to know what I'm thinking, as I'd like to know his thoughts also.