Considering wet driving alone,might be enough reason to keep it on.
Even though your notchback enjoys some of the downwash off the roof slamming into the trunklid,you may have rear lift due to the separated flow off the backlight header.
The spoiler can help with reattachment,killing the lift,and hydroplaning propensity.
Here you can see how a 1994 Mustang GT is harvesting momentum with their rear spoiler.
The Dodge Viper GTS reaches for even more air
