In my opinion it is far simpler to stick with one card and buy a new one every year or second year. More parts = more things to go wrong and more heat/power.
This and poor scaling...which I know it's been pointed out that the GTX 480 scales very well, but to me it's just not worth the cost or extra heat/power requirements. I'm perfectly content with my single 4870. I have a 22" 1680x1050 monitor and I personally don't see any need for more power. And by the time I do, even though a second 4870 wouldn't be very expensive, provided I could find one, for me, it's not worth the cost, effort, and added heat/power requirements of buying a 2nd 4870 along with a CrossFire motherboard and new power supply. Heck, that's halfway toward a new computer. Also, in such a case, I would have to buy a CrossFire motherboard and PSU ahead of time just planning to do CF down the road. I'd rather spend the money on a single newer graphics card that'll perform as well if not better than CF 4870's and will be quieter and produce less heat. Not to mention the variable results of CrossFire and potential complications of adding additional hardware. Why spend more money initially for an upgrade that, by the time I were to do it, would just be hot, noisy, and old tech? That's the beauty of computer tech; by the time the average user, even the average power user, is dissatisfied with his hardware, the technology will have progressed enough that it is actually just as cost effective, if not moreso, to buy a totally new GPU instead of a 2nd one.
There are only 2 instances in which I could justify CrossFire/SLI:
1. You're hell bent on getting a very high end rig, and the caliber of GPU muscle that you're looking to get can only be found in the form of 2 high end GPUs in SLI/CF (or a dual GPU card, which for simplicity's sake, is the same thing). Keeping in mind that this would cost on the order of $600 on GPUs alone (assuming 2x5870s today), I don't think I will ever be in a situation to do this myself. Not to mention the heat generated by 2 high end cards would kill me (I live in Texas and my room is already the hottest in the house).
2. You're hell bent on getting a high end rig, and the caliber of GPU muscle that you're looking to get is most cost-effectively found in the form of 2 (relatively) lower-end cards, such as could be said about 2 5750's or 5770's today. This is a much more viable situation in my mind, especially if you buy one of the GPUs today with the intention of buying the second one relatively soon. The problem is, if you get content with the 1 GPU, you end up with the same situation that I described in the last half of my first paragraph. Beyond that, I personally don't spend most of my time gaming, rather I do spend a lot of my computing time just doing general stuff like browsing the net, listening to music, watching movies, etc. It's during these times that a multi-GPU setup would be needlessly guzzling energy and heating up my room. Granted the newer generation of cards (from ATI, at least) is very efficient at idle states, it's still extra heat that doesn't need to be there, when you consider that, at this performance/price point, you could just as easily get a single-GPU card that is of approximately the same caliber as the multi-GPU setups for about the same cost, when you consider the savings in PSU and motherboard costs.
Shouldn't you just relabel "Never used with SLI or Crossfire" as "Sorry, I'm not an enthusiast" ?
I could simply try to insult you or make a sly remark regarding a correlation between SLI/Crossfire and having a small penis, but instead I'll write a (fairly) serious rhetorical response.
This is simply not true. Firstly, this assumes that you have a high-end multi-GPU setup, as any multi-GPU setup that can be matched in performance by a single-GPU setup would negate the status of the former as an 'enthusiast' solution, as an inherently 'non-enthusiast' solution could offer the same caliber of performance (again, regarding multi-GPU cards as multi-GPU solutions). So labeling any SLI/Crossfire setup as an enthusiast setup actually contradicts the label itself. Further, the situation of having a single card then adding another one months later would not make you any more of an enthusiast than someone who will simply keep a single high-end card. The only situation left is someone who spends a lot of money on multiple high-end cards, in which case you simply have a lot of money and/or a lot of time to put towards playing games. This doesn't necessarily make you more or less of an enthusiast than someone who doesn't have copious amounts of time and/or money, but, then again, if your life revolves solely around your computer, then I suppose it does. So yes, if you consistently have a multi-GPU setup consisting of high-end GPUs, I suppose you would be more of an enthusiast than people who would be running a multi-GPU setup based on older tech or, god forbid, has "only" a single high-end GPU.