What are the user advantages to a unified directory tree? I've always found drive letters to be a moderately useful abstraction (I've been tripped up a few time

What are the user advantages to a unified directory tree? I've always found drive letters to be a moderately useful abstraction (I've been tripped up a few times on my Linux server when moving 20GB+ video files around - if I forget /home and /srv are mounted on different drives, a move operation takes a long time, but having separate drive letters makes it more immediately obvious).