I woulda thought it could easily do threading too. Regardless what the speed of the spindle is, if you can get it down to 70 rpm, you control the bit, so you jump in, race across and jump out with your cross slide. But hey..I don't know anything about the sherline lathes. I know my mill will do threading or tapping or dieing. It's pretty simple, you just need a diameter, a length and a pitch and a depth. I guess buying a sherline lathe is out for me. I am not that impressed with their machines anyhow, I think the motors and the pulley's are weak. I've seen stronger rubber bands in my wifes hair than the belts on those machines.
There's the point about drilling too. You say the lathe will free up time for you to be drilling, but doesn't the sherline do that too? If it can't thread and can't drill, what else is there, just x,y motions for a tool bit?