Here's the deal...
First off, I dislike ordering online. I hate waiting for stuff and I dislike paying in advance for stuff.
Second, if the USB Adapter was sold in stores, I'd have a good chance of picking one up for a cheaper price - as likely someone would have it on sale. With it only being sold on Nintendo's webstore, it likely won't be on sale.
Third, I don't have a high speed internet connection - therefore, I don't have or need a router. However, with the USB Adapter, I'd have a quick and easy option when I go to friends/family house's who do have high speed connections. Just pop it in go - no set up required - wherever I'm at (including being at work, if it works!)
Fourth, I have to justify the purchase to my wife (Yeah, I'm whipped). Since I can't really use it at home - unless I find a deal on it, it'll be a hard sell. However, if I could even use my 10% discount on it, then perhaps I could swing it.
Fifth, I don't use my USB ports anyway and generally, my computer is on all the time. So if I ever *did* get High Speed (I'm somewhat looking into Verizon's DSL for $15 a month... It's hard to convince my wife we need that when we have free Dial-Up access right now...)