The memory real-mode/protected-mode is not really tied to DOS, but to the mode in which the processor is working. You can have DOS programs working in protected mode, as well as some operating systems (like CP/M?) working in real-mode.
That doesn't change the fact that when you write for DOS and want to run on every computer that runs DOS, you have to support the 8088 and 80286.
Isn't this page about maximal portability? Don't assume all the world's a Pentium.