Actually, we might just as well decide that it was a stupid idea in-universe
as well. Starfleet tried to achieve something with it, but failed, and was then stuck with a design that could only make use of two of the originally intended great many positions - fully up or fully down.
That is, originally the ship was supposed to have nacelles flat at warp 1, halfway up at warp 7, and fully up at warp 9.7 or whatnot. The impulse engine designers were told this would be the case, so they decided to put the impulse engines in a location that would get hot warp plasma in the easiest possible way - in the warp engine pylons. Alas, that meant the impulse engines could only be used with the nacelles flat - and this meant problems when all warp flight was later confined to the full-up position.
Personally, I prefer the explanation already suggested by Saquist
, that the squeezing action itself is the important thing, not the initial or eventual position. Never mind that the engines don't glow blue-hot during the squeeze: we can argue that they are at least starting
to glow during the squeezing action.