And the feat is nearly useless for him later?
It really depends on which part of the character's career we look at.
If we compare the value to the fighter, surely GWM is less for war cleric. But for one or maybe two combats it can be big early on. Remember, the war cleric is going to get more swings with a greatsword than anyone levels 1-5, right?
Some combats your role might be more at spellcasting. If a war cleric could kick but with GWM every combat the problem would be overshadowing the fighter.
I look at it like this: classes do not get extra attacks until 5th. From 1st to fifth, the war cleric is BETTER with GWM than many others. From 5-11, the cleric only has one attack but can get up to five extra swings from wisdom and cleric abilities. They can mostly go GWM as the dominant activity for at least one combat and be competitive with martials. When a fighter gets three attacks, GWM does more for them by a wide margin. But honestly, much of our groups play takes place before that.
By then, I am assuming the cleric is throwing larger spells a fair amount of the time as well.
If we compare the value of GWM for a war cleric at super high levels to that of a fighter with 4 attacks, then of course it is worthless. But this is a team game and through much of a cleric's career it can give an added boost.