I am a very rocket-spammy soldier, frequently shooting into big groups of enemies assuming it will hit something. As a result, the black box is well suited to my play style.
The one significant drawback to losing a rocket that hasn't been mentioned is related to rocket jumping. If I jump, I can easily fire two rockets before landing (see earlier: rocket spamming). When I do land and am presumably near combat, I already have an ENTIRELY empty clip. This has gotten me into pretty big problems in the past, and sometimes I do truly miss the regular rocket launcher.
Either way, I think it's a matter of personal preference based on play style. There really isn't a "right" answer, since everyone plays a little differently.