Also, nobody ever called grease useless. The worst it's been labeled is "overhyped," which is fairly accurate.
You are right, s/he didn't use the word useless.
The exact words were "the most overhyped", which is probably a gross exaggeration (even if the spell is overhyped, is it the MOST overhyped them them all?)
I think you're being told Grease is useless for this particular application because it only helps you escape a grapple, and if you need to escape a grapple that you started then you've necessarily wasted your time. And time is one of the most valuable resources in the game.
It is not the only thing it does.
You can use it to escape grapple, or you can use it to escape a pin while remaining in grapple. This means you can pin the enemy but the enemy has a very tough time pinning you.
The results of a pin:
http://www.d20srd.org/srd/combat/specialAttacks.htm#grappleIf You’re Pinning an Opponent
You can attempt to damage your opponent with an opposed grapple check, you can attempt to use your opponent’s weapon against him, or you can attempt to move the grapple (all described above). At your option, you can prevent a pinned opponent from speaking.
You can use a disarm action to remove or grab away a well secured object worn by a pinned opponent, but he gets a +4 bonus on his roll to resist your attempt.
You may voluntarily release a pinned character as a free action; if you do so, you are no longer considered to be grappling that character (and vice versa).
You can’t draw or use a weapon (against the pinned character or any other character), escape another’s grapple, retrieve a spell component, pin another character, or break another’s pin while you are pinning an opponent.
If You’re Pinned by an Opponent
When an opponent has pinned you, you are held immobile (but not helpless) for 1 round. While you’re pinned, you take a -4 penalty to your AC against opponents other than the one pinning you. At your opponent’s option, you may also be unable to speak. On your turn, you can try to escape the pin by making an opposed grapple check in place of an attack. You can make an Escape Artist check in place of your grapple check if you want, but this requires a standard action. If you win, you escape the pin, but you’re still grappling.
A pinned enemy can NOT cast dimension door because he can't speak (unless he has silent spell)
Escaping a pin does NOT end the grapple (hence the bonus is not useless).
A pinned enemy can't attack you, but you can attack them. That is, if neither is pinned, both can deal damage to each other during a grapple, if one side is pinned, they can take damage but can't deal damage until they escape the pin.
Also, the entire party gets an effective +4 to hit someone you are pinning.