For flashlights, Ti is basically just for bling. Of course Ti is more durable and corrosion resistant, but I have never heard of a robust Al flashlight wearing through. Ti is better than Al in seawater, but for the size of the typical custom Ti flashlight (1xCR123 or smaller), they would not be used for a main dive light anyway. I don't think anyone is really utilizing the inherent toughness of Ti over Al in a flashlight application. Maybe deflecting bullets?
Ti does have some considerable disadvantages when used for LED flashlights, such as much lower heat conductivity and cost. Also, Ti threads are gritty feeling and not nearly as smooth as Al or SS.
That said, my favorite lights are still Ti which goes to show that there is more to a good flashlight than just materials or pure output power.