my view on singability is that it does play into it on some basic level, but the crucial thing is, how singable is it in your head. and that comes down to pattern recognition, culture, folklore, as woggle pointed out.
catering to the human voice has a benefit in that it's more likely to touch upon similarities across culture, as that's the instrument we all use, and can easily hear in our head.
that said, huge cultural differences apply of course. good luck getting anything but a diatonic melody out of a random western whistler. except if it's really offkey
i got a lot of mileage out of trying to sing the stuff i sequenced when i started out. made me realise how hard it is to break some basic culture imposed patterns.
then again, some of it seems universal. like, a clean tritone being much harder to sing than a fifth. physics 'n shit?
but then you hear what great flavor it can bring to music, and it becomes part of your repertoire. it's very singable, of course. you just need to break that harmony safeguard.
you gotta distinguish between something that's theoretically singable, and something that's very unlikely to come out of a mouth, like a prog metal guitar solo.
imagine we'd all get a nanotech upgrade to our vocal chords at some point that makes singing yngwie malmsteen easy as pie. i'm quite sure it'd drastically change our melodic sensibilities over time. note i'm using the plural in a lowest common denominator kind of way.
(well, not really. That is portuguese, but we would rather say: "deixa o coração falar" (let your heart speak).
