User blog comment:MOTZI/Think About It - Marksman's fate/@comment-8802138-20140323113012

As Dooomninja said, there are items to get all three, and actually, a late game Marksman has high AS, around 50% critchance and at least one lifesteal item.

It is really weak not to have all three:

Sustain is no mandatory, but helps in duels and for surviving. Critical Strike is what makes Marksmen strong, imagine a mage with 250% damage on spells on a 1 sec CD. Speaking of CD, Marksmen can increase their sustain and damage overall by reducing the cooldown of their attacks with AS.

Imagine a Draven AA with bonus damage + crit. Still better than without crit. Graves still needs AS, the buff is just to tell him: Engage with the dash, don't run away!