Well it seems you are not reading enough.

It needs a FAT partition (which was done on N900 and N9), which fragments storage on the device. Issues with file permissions and security, back-ups, restoring to factory settings etc... The current btrfs layout gives us huge benefits there that would not be possible if we needed to have some rogue FAT partition.
It needs to unmount/remount it every time mass-storage is enabled. This means every application that might need to handle data on that partition needs to be able to deal with it disappearing. That is a huge job.
Just try to take pictures with mass-storage enabled in N9, will just not work for example. Although that is a minor annoyance. It is an example of what can happen. More evil things happen also.
It also meant killing apps in the background to enable it, which could cause data loss. It also meant sometimes it could not be enabled depending on what applications are open. Not to mention data loss issues/crashes with applications that don't handle it, 3rd party application not knowing what to do when the fs disappears. Not to mention all the data loss issues that exist with mass-storage currently. You know the remove safely button in windows?

A number where never solved for the N900/N9 but it shipped nonetheless since there was not really a better alternative at the time.

And although MTP has its own set of issues it actually covers the same use case.

We might have some mass-storage exports in a later stage as I stated (like for the SD card) however we cannot guarantee file exchange functionality on the same level as with MTP.

But bitching about missing mass storage because Apple cannot keep breaking industry standards because it wants to push its own vendor lock in is not productive and is not going to get you anywhere. Don't shoot the messenger.