At least with removable storage you elliminate the problem of losing all your data if something goes wrong with OSX because you was able to intall another storage device, install OSX onto that and then put the original storage device in a usb caddy and connect it to the mac to be able to still have access to your data.
With soldered on storage that is no longer possible and unless you regularly carry out daily backups, all your data is lost if OSX crashes due to system files going bad and your forced to do a complete reinstall which of course means you lose all your data.
All you need to do is look in the forums at mac owners complaining of OSX problems and people recommending that they reinstall OSX to fix the problem. Apple are now basically saying, if you do not back up your data on a regular basis and something was to go wrong, your basically screwed on data recovery but don't go complaining to us (Apple).
Backing up data is an essential part of owning a computer. At its most basic level on a Mac, it's as easy as attaching an external drive and allowing Time Machine to run continuously in the background.