It's an extra step along the way that is a potential problem, no matter how well it is supported.
That's like.. if you are backpacking in the wild, you can take one big rucksack and be done with it.. or you can take a few smaller ones. Why would you do that? That is, I'm sure there are some usecases for some people, but wast majority of people have absolutely no use for SD card, it's just something that can potentially cause problems.
People were saying that SD card support in WP was great, that google should look up to MS, but in fact, in reality, it was/is awful.