Are we talking about Iceborn? Iceborn adds a LOT of direct damage combat stats to an Ezreal. All 4 of his abilities scale with AP, an improved Sheen proc is amazing, and CDR scales into damage better on Ezreal than any other champion in the game. Not to mention the non-damage related utility of armor, mana, and a big AoE slow. Heck, you could even argue that the mana is damage because of Muramana, and the AoE slow lets you hit more Qs and deal more damage while remaining safe.
Also, according to http://www.probuilds.net/champions/Ezreal Ezreal doesn't even build Iceborn that often. The Trinity spike is just too strong to pass up.