It's not uncommon to run both Linux and MS-DOS on the same system. Many Linux users rely on MS-DOS for applications such as word processing. While Linux provides its own analogues for these applications (for example, ), there are various reasons why a particular user would want to run MS-DOS as well as Linux. If your entire dissertation is written using WordPerfect for MS-DOS, you may not be able to easily convert it to or some other format. There are many commercial applications for MS-DOS which aren't available for Linux, and there's no reason why you can't use both.
As you might know, MS-DOS does not fully utilize the functionality of the 80386 and 80486 processors. On the other hand, Linux runs completely in the processor's protected mode, and exploits all of the features of the processor. You can directly access all of your available memory (and beyond, using virtual RAM). Linux provides a complete UNIX interface not available under MS-DOS---developing and porting UNIX applications under Linux is easily done, while under MS-DOS you are limited to a small subset of the UNIX programming functionality. Because Linux is a true UNIX system, you do not have these limitations.
We could debate the pros and cons of MS-DOS and Linux for pages on end. However, let it suffice to say that Linux and MS-DOS are completely different entities. MS-DOS is inexpensive (compared to other commercial operating systems), and has a strong foothold in the PC computing world. No other operating system for the PC has reached the level of popularity of MS-DOS---largely because the cost of these other operating systems is unapproachable to most personal computer users. Very few PC users can imagine spending $1000 or more on the operating system alone. Linux, however, is free, and you finally have the chance to decide.
We will allow you to make your own judgments of Linux and MS-DOS based on your expectations and needs. Linux is not for everybody. If you have always wanted to run a complete UNIX system at home, without the high cost of other UNIX implementations for the PC, Linux may be what you're looking for.
There are tools available to allow you to interact between Linux and MS-DOS. For example, it is easy to access MS-DOS files from Linux. There is also an MS-DOS emulator available, which allows you to run many popular MS-DOS applications. A Microsoft Windows emulator is currently under development.