Quote from LeslieGilliams10 or so sponge are not useful enough to make it worth the trouble - and to empty out a temple only to find there is no sponge would be enough to make me want to kill someone. What a fabulous waste of time that would be. In short, a perfect example of why the temples remind me of a poor mod. Even on the worst day when I drop down to the bottom of a desert temple if the chests are only filled with bones or flesh it still is better than nothing, and, it required nothing more than a simple pickaxe and less than 5 minutes.
It's not really about progression - it's poor implementation - like someone at Mojang thought "Oh, this would be cool...." so they half bake it, get bored with it, then leave it unfinished as so many components of the game now have that same feeling.
I don't know... The first time I went into one of these monuments it was a helluva lot of fun. Enchanted a bunch of new armor, brewed the right potions, and set out. Swimming around the interior of the monument was a lot of fun, and fight against the Elder Guardian was a lot of fun. And more so than the gold in the middle of the monument I am in love with the dark prismarine. Now I have looted a few of these monuments in my world purely to capture all of the dark prismarine and sponges.