What are valid reasons for the continued use of MS-DOS?

Answer 1

A huge number of custom applications continue to run on DOS. If an application does what is required, and there is no need to upgrade, many small companies don't want the expense of rewriting and upgrading.

Answer 2

DOS, by IBM employee Tim Paterson. Paterson went onto work for Microsoft that was then using MS-DOS, Tim Paterson wrote some programs in VB and MS-DOS.
As DOS stands for Disk Operating System it is kinda needed. UNIX has DOS so so does every OS that uses a Disk or Platter or Tape or whatever.
Disk Operating System was often abbreviated to the three-letter acronym DOS (not to be confused with the DOS family of disk operating systems for the IBM PC compatible platform). The use of "DOS" as a suffix of operating system names was most common in the era when floppy disk drives were the predominant secondary storage technology, and command line interfaces the predominant OS user interface. With a few exceptions, this mostly pertained to home-/personal computers of the 1970s and 80s.
from wikipedia.
http://en.wikipedia.org/wiki/Disk_operating_system
So it is kinda a needed bugger. MS-DOS is non compliant with alot of stuff that kinda miffs the SOA. But who cares about compliance when 95% of PCs are one Corporation.
DOS, I love it, I love my TRS-80 BASIC cause I'm to geeky for my shirt.

Answer 3

MS-DOS can handle some things better than Windows.. Operating systems such as Linux and Unix, use there own kind of version of DOS.. similar but not the same, and they run with a command line interface to do almost anything.. and Linux runs really fast.. with more features.

Answer 4

Some older programs do still use MS-DOS
MS-DOS batch jobs can be useful
If you have a program written for Unix, you might be able to run it in the MS-DOS shell with some minor modifications