Al, oddly enough, the Northeastern US happens to be our second largest area of Heat Pump penetration! We realize that homeowners are not looking to keep their pools opened all year around. Rather, they are simply looking to extend the swim season, and to be able to be the FIRST to open their pools, and the LAST to winterize it. This comes out to about March/April through Sept/October.
All heaters loose efficiency when the temps are cooler. Most standard heat pumps will shut down when the air temps fall into the low 60's because of ice build up on the evaporator coil. Hot Gas Defrost options will melt the ice and allow the heat pump to continue working down into the 20's and 30's...albeit, at a lower efficiency. I would admit though, not too many people will be using their pools or spas when it's that cold. AND if you do want your pool/spa heated at those temps, I will cede that chore to a gas heater!

When you maintain your water temperature, the amount of heat lost overnight mostly to evaporation will depend on if you use a solar cover (20% heat loss), liquid blanket (30 - 35% heat loss), or no cover (50% heat loss).
Basically, a heat pump that is allowed to maintain a temperature (as opposed to heating on demand) has less work to do. Although the same heat loss and recovery statement is true with a gas heater, it is just so much more expensive to maintain heat, therefore the task of the gas heater is not commonly left on to maintain heat.
So, it is asked to increase the temps from a lower water temperature, in a quicker time frame. This is the environment for a gas heater (IMO). Perfect for spas and a pool that is not used frequently when it starts to cool off.
The Heat Pump, again, allows you to maintain the heat, efficiently, so that the pool is always ready to use. No more waiting for it to heat up and watching that gas meter just spinning away!
(sorry for the sales pitch).