If there is too much neck expansion upon bullet release, the necks will get work hardened as you reload or you could get premature neck cracking.
There is also gas blow back between chamber and case preventing consistent sealing, and formation of a carbon ring where sealing finally occurs.
I've read where some BR shooters had turned necks all the way down to 8thou thickness. Pretty extreme, but I'm sure it's part of their plan. Their chamber must be set up for that.
The only advantage in potential there is LOW neck tension. Low tension = low variance of tension, and a high starting pressure can still be obtained through bullet jamming. I might do that if I shot a 6PPC.
Most of the time, I turn necks. But not where that action would leave excess clearance.
I consider anything over 2thou clearance as excessive already.
I've had guns with more clearance than that, but there is nothing I can do about it, and I definitely do not need to turn brass for it.
So what would it hurt if you don't turn necks? IMO, nothing.
If you have enough clearance, and you cull out cases departing in thickness and variance, and mandrel expand in a pre-seating operation, You can make straight ammo with consistent bullet release.
You'll gain nothing with turning.
