It's just as good as a regular spear for a wall of spears. I saw a photo of a spear-wall formation by some reenactors—the first rank crouched, pila forward between shields, butt planted in the ground, the second rank standing with pilum poised to throw. No horse would charge that without lots of training that nobody bothered to give them. Horses were expensive, anyway, not the sort of thing you'd want to throw away like that. Of course, lancers could charge in relative safety if their lances were longer than the pila, as many doubtless were.

-Simetrical