[RobW]

This leads to many problems and very weird situations.

RAW: You CAN'T defend against a jab
That's because the jabber won't normally be engaged with you when it's your turn to move, and so the Shift and Defend option is not available to you.

RAW: You CAN defend against a jab, if a different enemy is also engaging you.
If an enemy has you engaged at the time of your movement, then you can select option (k) shift and defend. Now you can defend against the jab that is two hexes away (as well defending the against the enemy next to you).

RAW: You snuck up behind your opponent, but oh no, now you can't Defend!
That's because you were not engaged if you are in the enemy's rear hex, and so you can't select option (k).

You and your buddy have surrounded the troll, Buddy's is in the troll's front hex, you're in the troll's rear hex. Troll has initative and makes your side move first. You and Buddy stay put! You've got the troll surrounded! But now troll spins around to face you. Since you weren't engaged when it was your time to move, you couldn't select option (k).

RAW: Pole weapons are a good choice for a duel, but should they be THIS good?
In a duel, there is no way to defend against a pole weapon charge attack if the pole weapon does not want to allow it.

It's Halberd vs Sword, they start with 1 empty hex between them. Sword wants to Defend against Halberd's initial charge, but he can't -- he is disengaged when the moment of his turn to move, and so option (k) is not available. So he moves just out of charge attack range, 5 more hexes back. Halberd shrugs, charges to within jabbing distance and gets a free attack (no defense possible here either). Now we're back to where we started. In this duel there is no way for Sword to Defend against the initial charge if Halberd wants to prevent i t.