It only spawns imps if it crits. Honestly to me the 15second timer between ticks has become a little old, but no matter the period a dot ticks at the current system there will always be that scaled "phantom tick" at the end of a DoT unless you reach that magic haste threshold at an exact value (which will never happen).
So unless they completely disable the chance to proc on "phantom ticks" it could always potentially be better to let it fall off because 9 extra chances to proc over a 10 minute fight is still better than 0 extra chances even if they reduce the proc rate or scale it based on your Crit. The point where you'd actually lose a dot tick of damage wouldn't occur until the length of the fight extended to the tick period for Doom.
For example Doom ticks at roughly 14 seconds in the gear I currently have on beta. In order for me to lose one ticks worth of damage over the course of the fight the total fight length would have to last longer than 15 minutes because I'd potentially lose 1 second of up time per application of Doom. Therefore the only way to lose a tick is for the fight to last longer than the tick period in minutes.