You're exactly right. And I've said this for years...
It's not that the nacelles have to be "up" for warp drive
(as you say, just leave them up).
It's more like...why do they need to be lowered for impulse?
If the nacelles need to be in the upright and locked position for warp speeds, just build them fixed in that position.
If they need to be moved down, then that must be for impulse speed reasons, and what's the reasoning there?
Aw hell...it's just a case of wanting the ship to do something kewl and having moving nacelles.
The warp/impulse/whatever excuses are up to us to imagine.