Look, here's the reason reach weapons help wardens.
Scenario 1: Marked adjacent enemy attacks your ally.
Warden with reach: Enemy attack is at -2. Warden responds with melee attack vs reflex, that causes the foe to grant CA if hit.
Warden without reach: The same.
Scenario 2: Marked adjacent enemy shifts one space away, and then attacks ally.
Warden with reach: Enemy attack is at -2. Warden responds with melee attack vs reflex, that causes the foe to grant CA if hit.
Warden without reach: Enemy attack is at -2. Warden responds by pulling the marked enemy back into melee reach, after the enemy's attack is complete.
Scenario 3: Marked adjacent enemy takes a move action to get 2+ spaces away from the warden, and then attacks an ally.
Warden with reach: Enemy attack is at -2. Warden gets an opportunity attack when the enemy leaves. The warden also responds by pulling the marked enemy back into melee reach, after the enemy's attack is complete.
Warden without reach: Enemy attack is at -2. Warden gets an opportunity attack when the enemy leaves. The warden also responds by pulling the marked enemy back into melee reach, after the enemy's attack is complete.
So both wardens are exactly the same in scenarios 1 and 3, but there is a very big disparity in scenario 2. Notice that your enemy gets the most choice about which scenario you're in: If you have no reach, then as long as your enemy has the ability to shift one space away from you and attack your ally, the warden's mark is exactly as good as a totally vanilla monster's mark.
That's why one of the warden builds gives you all kinds of paragon feats for shifting before making your mark attack. The strength of a warden who can stop the "shift 1, attack" response to being marked is significantly upgraded as far as defending is concerned. If you're not that build, or if you want this ability at lower levels, then a reach weapon gets you it immediately.
Scenario 1: Marked adjacent enemy attacks your ally.
Warden with reach: Enemy attack is at -2. Warden responds with melee attack vs reflex, that causes the foe to grant CA if hit.
Warden without reach: The same.
Scenario 2: Marked adjacent enemy shifts one space away, and then attacks ally.
Warden with reach: Enemy attack is at -2. Warden responds with melee attack vs reflex, that causes the foe to grant CA if hit.
Warden without reach: Enemy attack is at -2. Warden responds by pulling the marked enemy back into melee reach, after the enemy's attack is complete.
Scenario 3: Marked adjacent enemy takes a move action to get 2+ spaces away from the warden, and then attacks an ally.
Warden with reach: Enemy attack is at -2. Warden gets an opportunity attack when the enemy leaves. The warden also responds by pulling the marked enemy back into melee reach, after the enemy's attack is complete.
Warden without reach: Enemy attack is at -2. Warden gets an opportunity attack when the enemy leaves. The warden also responds by pulling the marked enemy back into melee reach, after the enemy's attack is complete.
So both wardens are exactly the same in scenarios 1 and 3, but there is a very big disparity in scenario 2. Notice that your enemy gets the most choice about which scenario you're in: If you have no reach, then as long as your enemy has the ability to shift one space away from you and attack your ally, the warden's mark is exactly as good as a totally vanilla monster's mark.
That's why one of the warden builds gives you all kinds of paragon feats for shifting before making your mark attack. The strength of a warden who can stop the "shift 1, attack" response to being marked is significantly upgraded as far as defending is concerned. If you're not that build, or if you want this ability at lower levels, then a reach weapon gets you it immediately.