I would agree that mobility is always an issue – and like you say very much so in the lumbar region… for groin injuries and longer term retirement issues also (hip replacements etc).
Here’s another secret cow about to be killed – stretching is overrated.
Personally I think flexibility should be promoted in the gym not as an isolated issue but in proper ROM when lifting and a proper balanced lifting program.
Flexibility alone is useless as control is always necessary towards the end of ROM’s – this is not always appreciated.
Now – that’s not to say if something is tight don’t stretch it. No. But perhaps ask why is it tight? Is the tightness hiding an underlying issue in the training program? If we look at tightness from a neural aspect we would see it more as a protective mechanism rather than a metabolic process.
However in my humble opinion many hamstring injuries are caused in the stopping phase of movement because of residudal tension in the quads.