Scenarios are NEVER needed singly: they are always needed as a set. (typically,
at least 3 scenarios are needed: best case, normal, and worst case). 

But in Calc, the GUI does not take advantage of the fact that a given set of
scenarios will enter different values in the SAME cells. Instead, for each
scenario, Calc forces  the user to (a) change the values in cells all over the
sheet, (b) Select the entire range, and then (c) use the Tools>Scenarios... menu.

This is tedious. Besides, the user may make a mistake in selecting the cells of
the scenario, in which case Calc will use the default (earlier) value even
without the user's knowledge!

In comparison, in MS Excel, the scenario builder allows you to identify the
cells with variable values, then takes you in scenario-naming+defining loop, in
which you name a new scenario, and its values for all the identified cells. You
enter the new values directly in the GUI, unlike in Calc. You can keep adding
more scenarios without leaving this window. Once the set is fully defined, you
can leave the GUI.

This is far easier, and without any possibility of an error.

Coming to how the scenarios are used:

In Calc, you are allowed to activate each scenario one at a time and see the
picture. You cannot compare the scenario by putting them side by side. In
comparison, MS Excel allows us to build (a) summary that shows all scenarios
side by side and (b) Pivot table.

This approach is much more useful.

***
To sum up, method of building and using the scenarios must be changed.