Think again, when the internal storage is miniscule compared to the amount of data these devices create and use now. Wanna take movies with you? Nope. One year worth of photos add up to more than your internal storage? Nope. Want to download podcasts to listen to? Well, if you have any space left in your internal storage after the photos, maybe....
Here in Taiwan everyone uses SD cards. Also because when you take your phone to servicing (which it will inevitably need some point), can just take the SD card out, and you have your data. With internal storage you better wipe the mother before handing it in, because who knows who will be checking what you have on your phone..
As one data point, I explicitly did not purchase an HTC One because it did not have an accessible SD card. It is a key buying feature for me.
The main reason I like the external SD card is that I can bump up the storage capacity tremendously, which has all kinds of benefits. It also has other nice perks: changing between devices becomes easier, easier to use as a portable USB drive, etc.
I feel I cannot be alone in this, but I don't have any hard numbers for you, sorry.
The elephant in the room: it is used for porn. Porn drives many things IT. As it is still forbidden or restricted in many countries, most user cannot just point their browser to porn sites. So they download or copy pics and vids to SD cards. Porn on the phone is so convenient for students, young workers (because they do not hav e their own room with a good PC screen to "relax" alone).
Why do you think most (and best) Android file explorers are Chinese?
Seriously, Nexus are not even marketed or sold retail or with contracts in most countries, and the list of countries in where they can even be bought online is a very small list of countries. Samsung outsells Nexus 100 to 1 because they phones actually exist as a product someone looking for a phone can choose.
Motorola offers more storage than Samsung and almost all of their phones come with SD slots and is also outsold by Samsung 5 to 1.
That seems completely unreasonable, but in fact, one reason I like my Galaxy Note II is that it supports a 64Gb card.
(It sure ain't TouchWiz!)
It is mainly the large screen though for me, but the 64Gb card holds my music collection and some videos and a lot of reference books when I am in no position to stream anything.
I would love to see hard numbers on that, because it sounds like bullshit.
Most non-tech people I know have no idea what a Nexus phone is because there is no promotion for it. Those that have a Samsung phone got one because they knew it was popular, thought it took great pictures, and loved the screen.
My guess: pretty small.