If you do not stow the reversers upon reaching the taxiway you risk ingesting FOD and other stuff, as an A319 crew found out when they exited the runway in wintery conditions with the reversers still deployed. The low swung engines ingested de-icing chemicals, which caused smoke en fumes in the cabin, which led to an emergency evacuation initiated by the cabin crew (!).
Report: Easyjet A319 at Belfast on Jan 6th 2011, taxiway de-icing prompts evacuation
Anyway, Airbus FCTM says: stow reversers before leaving the runway to avoid ingesting FOD. Seems pretty sensible to me.