Dungeons & Dragons 3.5 was infamous for its esoteric and befuddling grappling rules. 4th Edition wisely makes grabbing your opponent something exceptionally simple, and the Crypt of Shadows encounter takes the chance to show it off.
Not to great effect, though.
The zombies in the Crypt of Shadows can attempt to grab the players. Their "zombie grab" is a +4 vs Fortitude melee attack; success immobilises the victim, and they can sustain the grab as a minor action. (Players can achieve the same effect with a Str vs Reflex attack.)
Immobilisation isn't paralysis; an immobilised character is merely prevented from taking move actions, and can still disembowel nearby zombies. In the unlikely event that some fancy swordwork is insufficient to fix the situation, you can escape a grab with a successful Athletics vs Fortitude or Acrobatics vs Reflex attack.
Grabbing is a tanking move; it pins the target into contact with the grabber and prevents them from going after more important enemies. In the Crypt of Shadows, there are no targets that are more important than others. Zombies who grab in this encounter are wasting an action.
This is a problem that has haunted roleplaying systems since the dawn of time, for one very simple reason. There is little reason to grapple an enemy if you are holding a weapon. If you are not holding a weapon, there is little reason to grapple an enemy rather than picking up an improvised weapon. In the unlikely event that unarmed combat actually occurs, there is no good reason not to resolve it using the normal combat rules.
The tanking uses of grabbing are interesting, but every class who is intended to tank has more effective ways of achieving this effect. A common theme is dealing damage as well as pinning the enemy. Players (but not zombies) may also drag a grabbed enemy up to half their speed with a further Str vs Fort attack, which might have some rare tactical use but is made mostly unhelpful by the sheer number of ways you can fail the attack or have your target escape before you reach your destination.
There is an example in a later encounter of the grab rules used well, as part of a monster special ability, but by and large, in an exception-based rule system, this is an exception we did not need to have.
 Would the grab rules be substantially improved by allowing a grabber to deal their STR modifier in damage each time they apply or sustain the grab? Or does this just make it clearer that grabbing is an unnecessary exception to the regular combat rules?