Though, if you're driving in the dark frequently, it won't hurt you! The dirt part ... I haven't cleaned my tires in literally years driving on paved and compacted (white stuff ... language barrier here) roads. If they do get dirty quickly, I'd clean them by means of a quick wipe.
To come back to the core question of reflective tires being effective. Argueable always, but I'd say yes. They do light up in poor lit conditions and are noticeable to other traffic.