Wall adapters are not available on non 1st class public transport or on the move, a USB3 port on compatible laptops can charge devices during sleep so yeah that's useful.
As for transfer speeds. We need a USB3 phone to test first of which there are none currently. But do you think Samsung will have put in a USB3 controller on such a device and then gimped it by leaving it at USB2 speeds? That would be pretty absurd.
For the record my 1st gen 64GB Sandisk Micro sdxc is rated to 30MB/s read and lower write. Using SD tester on the Note II it gets 10.1MB/s write and 18.1 read. You would not get 18.1MB/s reading to that card over USB2 and I just tested it in Mass Storage mode which resulted in 15MB/s read and 12-13MB/s write via my PC.
So the fact that the N2 can handle slightly faster internal speeds to the SD card vs over USB2 confirms the it's more than doable and is reasonable to say that a USB3 model of a handset will exceed USB2 speeds reading and writing to storage media.
If a 128GB card comes out at a fair price then I'll take advantage of the added speeds of the new gen model.
I don't think the Android apps that measure sd card speeds are very accurate. I used two different apps and got very different speeds. SD card tester and A1 SD Bench.
Results: A1 SD then SD tester. Galaxy S4.
internal sd 27 write/ 27 read and the other app - 12.5w/21.6r (2GB file)
external sd - 9.6write / 18.8 read and 12.3w/12.9r (2GB file)
I also copied a 1.07GB MS outlooks data file - it took 119secs to copy to external sd - so about 8.5MB/s
copied to internal memory - 59 secs - ~17MB/s write (a lot faster than one of the benchmarks)
My external SD card is a 64GB samsung (Transfer Speed: Up to 70 MB/s Read and up to 20 MB/s Write). So I'm a bit confused why it copied so slow to the extSD (8.5MB/s) when the USB port can handle at least 17MB/s.
extSD is on fat32 but I don't think that makes a difference.
I think theres a lot more to it like F2FS vs EXT4 that should improve speeds, not just upping the port capacity.
http://www.anandtech.com/show/7235/moto-x-review/9
So after my quick testing, if USB3 indeed unlocks performance on this storage I'll be very surprised. I don't know why we can't get USB 2 speeds of upto 35MB/s on phones. Looks like there's a lot of overheads slowing it down so it wouldn't make a difference if you used USB 3.