Commands are simple to renember and use.
DOS is a very stable OS. I only had it crash once.
DOS is the underlying OS on all Windows products today discounting the NT line (NT, Win2000, Win XP).
DOS does not take up that much space on a hard drive, (only around 8 MB for a full install).
Very portable (look at bootdisks, although the fuctionality is not near that of a full OS, it works great for rescuing a crashed hard drive, Fdisking, formatting hard drives, and running old DOS apps that will not run with Windows or newer versions of DOS).
DOS is a contraction for Disk Operating System.
DOS is very stable, I have only once had DOS crash on me.
DOS is cheap, and free if you go on the internet.
Very fast even on a reletivly slow machine.