[General Topics] Menneisyys:
1.2.1 Cases you will NOT want to use an MS-DOS emulator - I: existing ports on other, more Windows Mobile-friendly platforms

For example, if you want to play, say, the infamous Prince of Persia (which doesn't have a direct WM port), I would say you’d better get a NES (Famicom) emulator. The NES version of Prince of Persia is very similar to that of the PC; however, the NES version in no way requires as much resources (including battery power) as the MS-DOS one. Please see THIS for a complete review of NES emulation.

The same stands for, say, Xenon 2 or Risky Woods. Running them in a Sega Mega Drive / Genesis emulator is a much better choice than ding the same in an MS-DOS emulator. The Sega Mega Drive / Genesis emulators require less resources, have better sound (particularly with Xenon 2, which, on the IBM PC, only had speaker sound), are easier to control and produce higher frame rates (just as with NES and Price of Persia) than the current MS-DOS emulators. See THIS for a complete explanation.

These are not hard-and-fast rules. That is, it can't be stated "whenever a port of a game exists on a better-emulated platform, stick to it and forget MS-DOS emulation entirely". When selecting an original platform and an emulator to play a given title, you may want to prefer other, better-emulated platforms for action games like the above. For mouse (stylus)-based, generally, adventure and strategy games, on the other hand, you may want to prefer MS-DOS emulation. For example, compare the following two screenshots, showing Sierra's King's Quest 5 on a(n emulated) VGA PC (running the VGA version of the game) and on the NES (Famicom):

The difference in the visual quality is staggering, isn't it? And, the better visuals are just part of why the MS-DOS version is much more worth playing. The NES' not having analogue / mouse-like input, its cursor needs to be controlled by a (digital) D-pad. This means it's REALLY hard to control the game with the D-pad only.

Incidentally, the same stands for games running on other (mouseless console) platforms like the GBA version of X-COM. These kinds of games should be played under an MS-DOS emulator because they allow for stylus usage, unlike GBA emulators.

1.2.2 Cases you will NOT want to use an MS-DOS emulator - II: existing interpreters for a given game

As has already been pointed out, a specific subset of MS-DOS games have so-called 'interpeters', which run the games without emulating a full MS-DOS environment. This may be advantageous in many respects:

as they only need to run the game (and not an emulated operating system), they, generally, consume far less CPU cycles (and, therefore, both run flawlessly on slower and/or underclocked machines and result in a decent battery life). A well-known example of this is the Magnetic Scrolls interpreter (see review & tutorial HERE), which runs old, freely and legally (!) available Magnetic Scrolls textual adventures in a really CPU- (and, consequently, battery-) friendly way.

they, in cases, are much better integrated into the operating system. Again, an great example of this is the Magnetic Scrolls interpreter. It uses a standard text area (with some extras; for example, automatic text expansion of highlighted text) and standard, relocatable picture boxes. This also means it's, for example, possible to copy some of the text onto the clipboard. What is more, these operating system-level components are compatible with all kinds of operating system versions and configurations. This isn't always the case - let me point out that, for example, (W)VGA and square screens are not supported by many emulators (including, unfortunately, the most recommended one by n0p). Now, compare the quality of the rendering of the in-game text when done by the Magnetic Scrolls interpreter and n0p's MS-DOS emulator (not supporting (W)VGA screens). Yes, the difference is enormous - the interpreter-based version is perfectly playable even on a square-screen or a low-resolution (QVGA) device, while the original MS-DOS game rendered by n0p's emulator is fully unplayable on any Windows Mobile devices because of the emulator's not supporting high-resolution screens, while the games itself used EGA high-resolution.

Of course, it's not only the Magnetic Scrolls games that should be played in the related interpreter, but also a lot of other titles. For example, the earliest Sierra games should all be played under PocketSarien (review & tutorial HERE); most of the Lucasfilm games run better under PocketScumm than under MS-DOS emulators and so on.

There are exceptions, though; the most important being FreeSCI, the interpreter of (early) Sierra SCI games (released in 1989 and 1990). This is so uselessly slow that you will want to prefer an MS-DOS emulator to playing them under FreeSCI. (See THIS for more info.)