I've had this discussion with quite a few people and while I'm in the general minority on this, I'm happy to stand up and say that I rather like them.
Having said that...
I can understand why - where a digital option is available - CDs are preferable as a secondary medium. Presuming that usually CDs are produced in conjunction with a normal vinyl release (usually as the primary physical thing), they're then dead cheap to pop one in a record sleeve compared to arranging for a 3rd party to store and distribute (and charge for bandwidth of) a download version. And the other thing is that the better the download copy, the greater storage requirements and stress on servers etc. But, a CD that comes with a vinyl release will almost always only be used the once to rip to another storage device (computer, iPod etc) and then is simply a wasteful, useless blob of plastic.
I think the thing I like the most about download cards is that the potential is there to do something brilliant - as well as an almost limitless scope for additional content (an old trick - anyone who still owns a Dreamcast may/may not be aware of the "Omake" folders that come with their games), the cards themselves can (and SHOULD) be part of the overall art package. There's already evidence of this with some cards where a bit of thought has produced some really cool companions to the actual record, hopefully this will become more of the norm in time rather than the bland coded bits of paper that we usually get.
This side of things though is still in its infancy, and more efficient future generations of technology will make this a lot better (Neil Young's Pono might seem like an extravagant waste of memory space now, but is incredibly forward-thinking in the longterm) so this will change, and some labels already offer a choice of file formats to suit their customers' drive size. I do admit though that buying from Bandcamp can be a bit weird, when you get a download on purchase, plus another card on arrival - I find that these make ace little presents to fit in birthday cards as they not only spread the word of good music, they haven't technically cost me anything