For actual songs having a scalar resolution is important of course, hence
the use of solfege in traditional teaching. For myself, I use chromatic scales
because developing a 12 tone resolution is important for jazz.
How often do you hear singers siren, well, other than teachers trying to
impress you with their range ? Legato is VERY important, yes, but
doing a legato run on a specific series of tones and having the intonation
correct is also crucial, and you wont get that via some uncontrolled siren.