In the simplest sense, you should use one because:
- we're asking you nicely to do so,
- there are plenty of other on-line places which allow mechanics-less
gaming, and
- we believe having a rules system is integral to good role-playing
gaming.
In a more complex sense, mechanics are like a mutually-agreed-upon
language in your game. Within the framework of that language, a
consistent story can be told.
Which mechanics system is used is entirely up to you -- use the one
you're most comfortable with. It is possible to have players that don't
speak the chosen mechanics "language" very well; the GM merely needs
to be very clear and patient with them as to what's going on. However,
should the GM not speak the language well, disaster looms. When the
GM -- the leader of the game -- doesn't know exactly what's happening,
how can anyone else be expected to understand the situation or to take
it seriously?
I personally don't want to hear about the way the mechanics work. But
a GM that knows (mechanically) exactly where everyone is and what they're
doing is a GM that understands the relative abilities of the (N)PCs,
and will keep things running smoothly and consistently.
Example:
I was in a a game that was described as "mechanics-light," i.e. there
wasn't any rules system for resolution of conflict. One of the PCs was
characterized as "a fantastic negotiator and diplomat," whose skills
far superseded the player's. The events of the game, however, failed to
support this claim, as there was no consistent mechanical resolution.
The player's skills could not adequately model the PC's. In the
absence of something like dice-rolling, most of the (N)PCs either
completely ignored the PC, reacted with hostility and annoyance to the
PC's presence, or simply denied she had any ability in her skill-set. The
PC had no consistency in her skills because, at any given time, everything
depended on the mood of the person running the character she was talking
to -- not the PC's skill set.
A good set of mechanics would have solved this dilemma -- just roll the
dice and do a skill test. However, when you have no definitive measuring
stick for skills it's far too easy to fixate on some small irritant
within the game or your personal life, and make that the (usually quite
logical sounding) reason the PC never quite succeeds.
Unfortunately, "people skills" (as a single example) have very
little objective measure. You know exactly how far your character
can jump, or you have an intuitive grasp of damage and defenses and
other physical relationships, because you can go out and measure them
personally. However, other than "pass/fail," how can we measure "social"
success in real life? There are no Socialization Olympics.