Do you all find it necessary? Why or why not?
The PHB available on Roll20 is a time-saving feature. It allows you to drag and drop race and class abilities (currently level 1 only) and spells onto a character sheet. You can also just type them yourself. Down the road, the higher-level abilities should be available as well. If the creator of a game has purchased the PHB, they can share it with the others in the game (more sharing at paid subscriber levels).
Just a note to people potentially posting a response: Please keep this focused on Roll20-related points and commentary. :)