Disarms and Reach
For GURPS at least, the Reach on Disarms is one reason to use it – because you can disarm someone from the combined Reach of both your weapons. This makes it very useful when approaching polearms, or as part of teamwork against an outnumbered opponent, or with long-weapon tactics. Simply stand out of striking distance constantly disarming until you either disarm or unready their weapon, whereupon that is the opportunity for your allies to rush in.
This can work in formation combat where spearmen with long spears stand 5-6 yards away and try to rake at each other’s spears before daring advance into stabbing distance, or in a duel with longswords where you can attempt to disarm from 4 yards away. The key advantage is you can do a disarm without being close enough for your opponent to hit.
Even an unsuccessful disarm can still be tactically useful if it provokes the enemy to rush in at you – Committed Attack, All-Out Attack or Move & Attack all lower defences, creating a window of vulnerability that you and your comrades can exploit. Meanwhile, the disarm does not require you to lower your defences at all, therefore you can still use Retreat and/or Cross Parry and/or Defensive Grip to make sure your defence against their rush is still good.
The disarm penalty is prohibitive when already in striking distance of your opponent; the penalty is simply too large compared to aiming for them, and so you put yourself at a disadvantage. The more reliable tactic to disarm when close is grapple (armed grapple?) and then opposed ST checks to attempt to pry the weapon from their hands. Even that is risky, best done from surprise where you ambush someone who has a weapon while you have none, but preferable to trying to fist-fight the sword-armed guy.
Actually, attacking someone’s hand is typically easier than attempting a disarm – and that has the advantage of making sure they stay disarmed. Similarly, for an unarmed option you might find that after grabbing their arm Arm Lock will be just as effective or more so than the opposed ST checks… unless you can’t afford to get bogged down grappling them. But if you’re outnumbered while unarmed by guys with weapons, you probably should have avoided this fight.
There is another benefit to disarms in GURPS; there is no DB from shields against them, so if you’re fighting against a large shield you can use this to minimise the effect of their high defences. This can be important psychologically if your opponent likes to rely on DB 2-3, as a further encouragement to push them into a rash advance.
So disarm can be very useful before closing, with room to back off, in combination with long weapon tactics. The key thing not to do is stand within stabbing distance while you do it.
Technically, even with Reach 1 weapons you could use this tactic to an extent. If you Retreat from their attack, rather than stepping forward into range to strike back, you could disarm and take another step back.
Disarms can also be a viable counter against polearms, not just using them. If our side has Reach 2 greatswords and their opponents have Reach 5 pikes, the greatswords can still attempt a disarm from Reach 7. Defensive Attack or Defensive Grip can be used to have a good Parry while still presenting a threat that the enemy has to react to, namely the threat of losing their weapons. Once the enemy with polearms step forward to attack, then unless there are supporting ranks behind them that can give the fighter with the shorter weapon an opportunity to rush in safely and move past the reach of the polearms.
Essentially, Disarms can extend your Reach and that opens up a lot of tactical flexibility.