Alternate picking give that raspy, vulgar sound that shred guitar may need. If you listen to John Petrucci, you can kinda of hear a very small and fast clicking sound from his solos. It sounds the pick, but its actually the strings being hit in such a way. That click makes such a powerfull effect when you end the last bar with an accending alternate picking run.

Its hard to substitute things for other things. You can replace tapping arps with sweeping. And picking tapping sounds very different than finger tapping. And Legato and alternate picking are two worlds are their own.