A lot of this 'discussion' depends on how you define 'efficiency'.
A 'point source' can be very efficient, in it's self. It can also be
very inefficient when compared to another type 'source'.
It's true that any antenna can radiate all of the signal getting to
it. The 'catch' is just how much 'signal' is getting to it and how/
where is it being radiated. If it's going to where you want it, and
if a usable amount of 'signal' gets there, then it's efficient for
that particular situation. If not... then it isn't very efficient, is
it?
- 'Doc

(With the 'proper' mind-set, you can apply the above to anything, not
just antennas.)