William; I've never had a lathe with this feature. Is this lathe a Variable Frequency Drive (VFD)? Metal lathe?
If turning / threading up to a shoulder, you want the lathe to stop RIGHT NOW. This is quite likely when making kit-less threaded pen sections. The common way around this is to turn as close as you can to the shoulder, then use a parting tool to plunge in and clean up the corner. This doesn't work for threads though.
However if your lathe has a threaded spindle nose on it, short decel may cause the chuck to unscrew. Some chucks will have set screws in the base of the chuck to prevent this.
The only likely reason for long deceleration would be if the work piece is fragile or has a very thin cross section at some point.
An intriguing question. I'll be interested in others comments.