Low speed wouldbe usefull if you are lifting heavy stubborn fish like sharks, large tuna, giant trevally etc.
I wouldn`t consider using a 2 speed reel with anything under 80 pound line.
Just my opinion, others will differ.
Some are using large spin reels like the Stellas for big fish, and they are single speed.
I imagine it won`t be long before we see 2 speed spin reels.