I recently found the need to add spells from a module that is not purchasable through the Marketplace. I needed to add the spells to a lot of different NPC's sheets. Unfortunately, without being able to create my own spells in the Compendium, that I can drag&drop onto any character sheet, this became a tedious and time-consuming task. If I had the ability to add my own spells to the Compendium, it would have taken a few minutes to add the spells to all the character sheets. Instead it took well over an hour, especially since I had to keep stopping to correct typos. A Homebrew Compendium is, IMHO, an absolutely necessary tool if Roll20 is to keep its subscriber base since this sort of thing is available on other VTT systems.