There's normal inertia and the rotational variant to look at. If you are capable of locking the rotation (due to a good grip with a strong hand) the enemy is fighting against the total sword's weight, to which the pommel contributes, when he strikes. But with the weight distributed either side of your wrist-located fulcrum there's less effort for you to spin it than all that weight at one side (give or take the nature of the distribution).
Of course it complicates the matter of technique, but I'd rather it that way than no pommel giving weight meaning all the weight I have to resist an onslaught (or, in a stike itself, to resist deflection or full on deceleration through blocking) also needs to be swung.
But, it depends on the weapon, and the fighting style. A lump of metal at the end of a shaft held only at the far end (possibly with a strap to allow some form of finesse) has its uses, much as does a quarter-staff has which is quick to 'whirl' and direct (if used with skill behind it) and has some oomph behind it despite not having the same enormous amount of intrinsic mass. Nunchucks are either the best
or worst of both worlds, depending on what practice you have (Youtube videos doubtless abound to demonstrate the various epic fails possible in this case!).
But I speak with only partial experience in the above, because I can't claim such broad-spectrum skills that I have experience with
all the weaponry first-hand, but I know what others say about (and do with!) the kinds I claim no personal domain over.