Well, some people are paranoid about the internal battery dying... I haven't had that problem yet, but I've had several devices which suddenly developed bad blocks on their internal storage, so I'm a bit paranoid about that.
Flash memory unfortunately still dies easily depending on the number of write access it gets. And unfortunately an ereader has quite some activity in that regard as it updates its database of books and read progress all the time.
With the SD card there is little to no downside (maybe power consumption), you get more storage and its easy to replace.
Also it's just nice if I can take the card out, put it in any other device with a card reader (tablet etc) and continue reading there, without having to resync everything.
So for you there may be no reason to use it; for me there's no reason not to use it... each to his own
