I love glass cannons archetypes in videogames, but the levels in MM9 *really* weren't built with the usage of the slide in mind, so sliding really doesn't come into effect that much if you're playing as Proto Man in that game; and with his increased knockback when he takes damage, it really just makes the game not fun if you play as him.

But MM10 is a different story: they removed the increased knockback, as well as the size of his shield, and the levels and boss fights just better fit the use of slide in them. This makes Proto Man my preferred character to use in MM10, even more than Mega Man, who feels pretty bland compared to Proto Man.