MarkT:
Vth is nothing to do with a mosfet being switch on. A Vth of 5V means it is fully off at 5V or below.
Hello there Mark,
I dont understand the reason for your statement. The states 'on' and 'off' are binary in nature, so if you have one you dont have the other, and if you have the other than you dont have the first. In other words, if something turns off at 5v then it also turns on at 5v. If the voltage is falling through 5v then it turns off, but if the voltage is rising through 5v then it turns on.
For your example, would you use that transistor in a circuit with 5v logic? And how do you know that?
Sometimes Vth is used to show when the device will turn on. In fact, it is sometimes used to show the minimum voltage that will turn the device on at least a little. Yes, it is better to know the typical Vgs and 'on' current level but Vth does tell you something interesting too.
If you prefer though, then just look at Vgs vs Id or the like.