Usefulness = (Functionality x Availability) - Burden
Functionality is what the thing does for you, availability is a number between 0 (for none of the time) and 1 (for all of the time) and burden is what you have to do in order for the thing to function.
It follows the formula that a most useful thing would be;
multi-functional, reliable, portable, low maintenance, and low cost.
An example. A grandfather clock and a wrist watch both have the same function - they tell the time.
The wrist watch is more useful as, wherever you are, you can look at your wrist and there it is. So availability is far higher.
One at a time.
A watch is one of the many things that you can only use one of at a time. So your first watch will be very useful but a second would be less useful so and a third would be even less ...
When the benefit of an additional item is outweighed by its burdens then it is too much.
