Even though it’s a small market today (although don’t forget about the iPad 3), it’s inevitably going to increase substantially in the near future. Don’t you want to get ahead of that? Do you want your site to be ready the first time someone views it on a Retina screen, or are you OK with it looking like garbage for a few years until you happen to buy high-DPI hardware?
If you don’t frequently see your site on a high-DPI screen, you may not realize quite how bad it looks when any graphical assets show at 1X. This is the first problem you need to address.
One of the first things I did when I got my third-generation iPad was to update the graphics on this site. Although photos, some layout, and news graphics are 1X (mostly), it is important to get ahead of the curve of high-DPI displays. The market share may be small now, but it will inevitably be the future. Eventually I may rework the design to take advantage of high-DPI displays, but right now the 2X graphics is the easiest first step.