For racing TTs, especially on dual carriageway couses and longer races, I put a pair of cateye rapid X2s on the rear stays. Shorter local races I just use one.
I have no evidence, but iIsuspect air flow around the seat post between the legs is more easily disrupted by a light, than around the seat stays. And more of a faff to fit. However, the trade off for me is visibility and warning drivers behind. Its a choice.
I have seen some very effective seat post lights though.
Just my 2-penneth.