Slow (SFTP) and very slow (BT).

That being said, I connect my N9 in mass-storage mode only if I have to transfer >10GB of data.

Apart from speed, mass-storage is still very useful when you need to use your phone as a USB flash memory, especially with devices that have no idea of what MTP (which is also far slower than direct USB transfer) is like many 'smart'-TVs and such.

I don't understand the mounting argument - I mean I understand it, I worked on a custom FS so I have pretty good understanding on what's going on beneath - I'm just saying it's not an argument. Most modern file systems (and storage devices) support journaling with transactions, advanced queuing and smart cache so the argument 'you need to unmount a partition to make it available to other systems' is pretty moot. There is pretty much no reason to limit file access to one host only these days - FFS, databases have been going at it for decades now - so why not have it both ways?

Well, apart from legacy reasons and "don't fix it if it ain't broken" mentality (and it is broken in this particular instance). Sure thing, allowing such thing won't make the above mass-storage problem (legacy) solved, but there is no real reason why ext4 cannot be extended (it just needs a bit smarter journaling) to allow mounting by multiple hosts. Then you can stick an ext4<->FAT translation layer on top of it for legacy purposes and be done with it.