First, I advise folks to, in general, avoid getting even close to maximum wire fill. Naturally, a lot depends on the specific of the run - but I find it doesn't take much to make for a hard pull - especially if there are LB's in the run.
Now, for your real questions: If you're not required to run a wire at all, can you run a small wire? Is a small wire better than no wire? Is a wire any real improvement over using the pipe as your ground?
Pipe makes an amazingly good conductor- even when the connections are loose or otherwise flawed. Still, it has limits; over a long enough run, even a perfect install will not clear faults. While the NEC does not consider this fact directly, it's pretty glear if you look at the GEMS calculations.
In some areas, some 'professional circles,' the use of the pipe as the ground is simply assumed to be poor, unreliable, and inadequate; wire alone is considered 'good.' As best I can tell, this attitude is not supported by either the NEC, nor the actual test data.
But isn't 'more' also 'better?' Not always - especially when that additional wire starts cluttering up already busy switch boxes.
When would I want, as a design choice, an additional ground (the wire)? For long runs, for complex runs, and for runs where the pipe is (let's be realistic here) going to get damaged.
Otherwise, I'd do without the wire. Pipe fill aside, even an undersize wire still adds to the clutter in every box.
That's my take.
