Well, for me (speaking in D flute terms) I regularly use all three octaves of the “D” fingered notes on my current C flute. Particularly for accompianment of singers that flexibility is essential. Low sonerous harmony and then for instrument breaks I often mix it up by jumping up an octave.
Clark
I have a C Olwell bamboo. Being cylindrical, it’s a bit of a bear stretch-wise and the 3rd and sixth are smallish and offset some.
I look forward to trying a conical one to compare the stretch and voice.
I play a lot of non-Irtrad tunes on it. It’s always easier to play a keyless flute in the key in question than to use keys. Keys are only really great for true “accidentals”. In other words when your playing a song in G or D and all of the sudden there’s an F natural. That’s what keys are best for in my opinion. Otherwise you’re always better off picking up the corresponding diatonic flute.
The above, of course, pre-supposes that a person is playing by ear and not using sheet music. A person tied to written music would have a nasty time transposing all those diatonic sticks and their fingerings. Yet another great reason to develop the ability to play by ear. 
Doc