Once the spawner's model starts spinning, the model will never stop moving again. It will twitch rapidly back and forth instead.
This is most visible on Ghast spawners (and might not be noticable on spawners for any other mobs).
This is visible in many youtube videos of CTM map playthroughs. See http://www.youtube.com/watch?v=pqsrB-4xC8s#t=23m52 for an example.
Comments

Is this still a concern in the latest Minecraft version 14w04b? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.
Even is the model has never moved- i.e. a player was never is range in a mode that wasn't spectator- the model will still jitter. It just becomes more pronounced once it has moved.

Confirmed for
1.8.5
1.8.6 Please add to the report the fact that this also happens if the spawner is outside the worldborder (see MC-55599)
Duplicated by MC-80628 (sorry)
Stuttering caused by player not in range
/setblock ~ ~1 ~ minecraft:mob_spawner 0 replace {EntityId:Zombie,MaxNearbyEntities:1,RequiredPlayerRange:0}
Seems to be caused also when entitiy rotates very slow:
/setblock ~ ~ ~ minecraft:mob_spawner 0 replace {EntityId:Giant,Delay:1000}
I found a video for version 1.3.1 happening this issue. Perhaps it's oldest video.
https://www.youtube.com/watch?v=BQu7Xh-cDpY
Can see a stuttering and flicking zombie spawner at first scene of this video.
Also I'm tested in 1.2.5, this issue isn't happend.

Confirmed for
10.1.2
Only if the spawner is outside the worldborder
Should I create a new report for this bug?
Yes, please.
I've noticed this myself. To clarify: this occurs when you are in range of a Monster Spawner and then go out of range, so that the flames and spinning should stop.