Stronger in fight if it has reached his higher forms, yes. Better as a summoning? No. By that time he's near being uncontrollable already. Besides, i wouldn't put my money on the eater vs. the horsemen too soon, with the poor protection of the eater light lances hurt. And if we're talking eater vs. another summon not overall usefulness you can just summon a wight or two and let them kill the eater with a baneblade.
And i defninitly don't remember the eater being a really awesome summon in Dom 2, though he should've been if he's supposed to be decent in Dom 3 with twice the cost.