Recently I was involved in a discussion about the (un)usefulness of a prepromoted unit. I argued that I prefer growths over bases.
In most cases prepromoted units have:
+ decent bases
+ good weapon level
- lower bases than a rised level 1 unit of the same class
- worse growths
However in FE9 prepromoted units have average or below average base stats, but great weapon level combined with good up to amazing growths. Their growthrates will equalize their low bases later on.
Except on higher difficulties I try to avoid using prepromotes as much as I can. The only exceptions are some units in FE9, Percival in 6 and Seth in two chapters in 8.
I go for bases in the early game, but for growths in the long run.
PS: I'm not a LTC player.