When deciding between Surf, Hydro Pump, and Scald, you've gotta take into consideration several things: firstly, does the Pokemon actually benefit from the burn? Starmie, as an example, could possibly benefit from the burn. It could cut down the attack of something trying to Pursuit it like Tyranitar and then Recover off the damage. However, if the burn doesn't occur, Starmie is royally screwed into spamming Recover to stay alive or try for Scald again for a parting shot at Tyranitar. Surf/Hydro Pump provide power, albeit without the power to burn and a slight drop in accuracy in Hydro Pump's case. The thing with Scald is, imo, a Pokemon that should use it is a Pokemon that can not only use it effectively to take out potential threats, but should have the bulk to be able to use it until it becomes effective (burns them). Suicune and Jellicent are perfect examples of what I'm talking about.