Serpa holsters are unsafe and not allowed by several instructors I use.
That being said, anyone in a rush to holster their weapon is a fool. If the threat has ended why is one In a hurry to holster? Take your time and make sure your shirt, etc. Is out of the way.
I don't know that its a matter of being in a rush to holster, but rather the requirement to safely and easily holster a weapon with one hand. That need could be there for and endless number of reasons, but some examples would be...
1.) holding a flashlight
2.) handling a handcuffed suspect during an arrest
3.) trying to brace or support an obstacle that could be blocking a path or falling/leaning dangerously, etc.
4.) possibly trying to support yourself, trying to avoid a trip/fall or to maintain your balance.
Obviously I can't speak for everyone... just my opinion.