In Mass Effect 3 if Legion survives the second game you find him in a Geth ship boosting Reaper codes against his will, but even if he doesn't survive they make a remake of him as a form of VI. But why do they have to use him, wouldn't any Geth work?
This is possibly because Legion [is built differently from most Geth:]
Legion is a unique geth mobile platform, designed to operate outside the Perseus Veil and interact with organics. To that purpose, it houses 1,183 geth programs, as opposed to the roughly one hundred found in other platforms (...)
My guess is that Legion's unique hardware are more suited for this task than a regular geth husk.