If a steerer tube is a little too short for the head tube,are spacers really necessary because the stem still fits ok?

Don’t understand the question…the steerer should extend past the upper pinch bolt on the stem, regardless of whether you have spacers fitted or not. The steerer should be a minimum of 2mm below the top of the stem or you won’t be able to preload the headset via the top bolt in the stem cap. If its too long, (and it often will be once everything has been put together) you need to add spacers, either above or below the stem.

spooky, I think OP means an extender for a too short steerer rahter than spacers for a “too long” steerer

edit: no he doesn’t, he means steerer is just right without any extra spacers – which is just fine