Inclined to agree with sub55. For one thing, if you look at accident statistics, in general terms the problem for cyclists is drivers turning across our paths when we have priority - i.e. pulling into or out of side turnings, or across us at roundabouts. I see no particular reason to assume that any risks in time trialling are different (although the CTT may know differently - most riding is in town, and most racing isn't). Even on dual carriageway courses, roundabouts must be a major issue.
As a result I can't see why anyone would carry a rear light and no front (because the threat will, more often than not, come from someone in front of you). I can see why you would carry a front and no rear (which of course is what cyclists did in the early days), but arguably if you need one you need both.
Not only are there risks in making rules that suggest negligence (compare the role of the CTC in the 1940s debate on mandatory rear lights), but there are even greater risks in making rules that encourage the use of the wrong single light.