This is a rather common inquiry in contests that require computing elements of a sequence.
For example, consider the following recursive implementation of the factorial function (Python):
f = lambda n:n<1or n*f(n-1)
This works well for all positive n;
f(n) returns the required result as an int.
f(0) will return the result of the comparison
n<1, which is the the bool True.
Fixing this is easy enough, but it costs a few bytes:
f = lambda n:0**n or n*f(n-1)
While it is possible to think of the Booleans True and False as numbers (usually 1 and 0) and use them as such in languages such as Python, languages such as Java or Ruby do not even allow casting a Boolean to a numeric type.
Since this is such a common scenario, I think we would benefit from a default. If a challenge doesn't contemplate returning Booleans instead of numbers, should it be allowed or forbidden?