The word 'FUNCTIONAL' is paramount to this discussion and I believe the majority of the debate revolves around the ambiguity of the word. If you are not 'FUNCTIONAL', you are 'DISFUNCTIONAL'.
Looking at the Merriam-Webster site, this definition would seem most appropriate:
2 : used to contribute to the development or maintenance of a larger whole ; also : designed or developed chiefly from the point of view of use
Therefore, the quality of being 'FUNCTIONAL' is dependent on the purpose of use. Thus, it can be said that you are 'FUNCTIONAL' at attaining maximum mass, while 'DISFUNCTIONAL' at maximum cross-sectional "powerlifting" efficiency.
However, we then run into degrees of 'FUNCTIONALITY'. Nothing is black and white. To really assess one's 'FUNCTIONALITY' one needs to determine the fiber makeup of the muscle and then run a whole battery of tests to really know what that muscle is "made of."
A common question in the debate is whether an athlete can perform "real-world" tasks. That is a viable definition for 'FUNCTIONAL' but it is not limited to "real-world" tasks.
If you want to make a claim to 'FUNCTIONALITY' of an athlete, you have to look at the sport in which he competes. Thus, Ronnie Coleman is the most 'FUNCTIONAL' bodybuilder. Lance Armstrong is the most 'FUNCTIONAL' cyclist.