the only real reason to have them on a smartphone would be for either music, video or photo storage. here are a few of my thoughts:
an android phone makes a pretty poor substitute for a standard m3p player unless you are tethered via usb or charger. the phone can drain its battery in a half day or less easy while my ipod can last 11 or more days. when battery performance is increased perhaps the device will be a good solution.
you have access to pandrora, last.fm, etc on an android phone. you are not required to have any music on your phone. in fact it provides an escape from your finite library.
apps do not take up much space. i had about 50-80 apps at one point in time and i didnt even fill up my internal storage let alone the 16gb card i have.
i can see storing some media on a phone but the device just isnt large enough for comfortable viewing hour after hour. tablets seem to be better suited towards this task. in any case, streaming services or youtube seem to be the majority of video on these devices instead of converting video files to fit.
as far as photos go, even at 3.5 to 6mb per photo the amount you can store even on a 16gb card is ludacris. that said, managing a photo collection of that size is easier and better suited for a laptop or desktop. we just dont have the tools required on a smartphone (yet) to make it easy. also, what is the point in carrying around some 4500 photos with you everywhere you go?
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i agree with your point that sdxc cards should be put into future released phones.
however, there isnt enough push to force manufacturers to head in this direction. the only answer for timeline i can give is "we will see it when we see it"
as far as reasons... i listed a few reasons why current cards are large enough for the masses and why manufacturers may be holding out.. but are all opinions.
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