With more and more externals using USB3 the case for them as being slower doesn't really hold up anymore, IMO. Besides, and maybe I'm wrong about this for the PS4 but at least on the PS3, sure you can swap in a new one yourself, that's all well and good, but then it's stuck in there for good. With an external option I can take it with me anywhere, it's a giant memory card now. If the XB1, which lets you install to the external drive, then lets you take all those installed games and be able to access them on a different XB1 just by logging into your account how is that not a clear advantage in this case?
Oh but you can just download them on a different system, sure, if you want to take the time and do so, maybe that 2nd system is located in a place with a slow internet connection? Or no internet connection? I thought shoddy internet was a problem right? So we can't be too sure we can just access and d/l our games again. Plus connecting a external to a usb port and powering up the thing is going to be way faster than any redownloading of your games.