WRT speed, putting a fairing on a high/low racer isn't the same proposition as putting one on a bent with more upright seating. With a very reclined position, you're already streamlined, so adding a fairing only serves to increase your frontal area. But other pros and cons still apply.

Pro:
protection from *light* rain, winds, sun, and cold, plus the 'cool' factor

Con:
added weight & complexity, harder to transport, can affect handling in crosswinds

Don't forget, having a fairing to protect from rain means the rain gets deflected - usually into your face. Some people dislike this more than getting their clothes soaked.