It handles some low-level GUI stuff (say, screensavers for instance ;p), but it shouldn't be critical (and I don't think stuff complains too much if it's down), and you can always restart it when you're done

.
I *think* (been a while, don't remember the exact upstart depgraph ^^) it's part of what's killed during an MRPI run, for instance, and KOReader also hobbles it during its own runtime.