Perhaps the Flight Crew Training Manual might be of interest.
A320 FCTM NO-110 (Approach): "A speed below the manoeuvring speed of the present configuration may be selected provided it is above VLS. When the ATC speed constraint no longer applies, the pilot should push the FCU speed selector to resume managed speed."
The disadvantage, of course, is reduced margin in case of unexpected turbulence (wake vortex encounters aren't unheard of on approach...) or perhaps the next flap selection being made in the wrong direction!
In the after takeoff scenario, once in the clean configuration, you're only likely to want to deviate from the managed speed profile to achieve either best angle (green dot) or best rate (considerably faster than green dot speed). As Airbus put it: "Pilots should be aware that it is possible to select and fly a speed below green dot, but there would be no operational benefit in doing this" (A320 FCTM NO-060 - Climb).
Last edited by Down...2 greens; 5th May 2013 at 11:46.