Yes, 'valid' has become accepted in common parlance as meaning just 'useful' and even in science, as you note. It is a bit like 'folk', in that it is just too short and handy not to be co-opted.

In the context I objected to it up ^ there, I think it WAS used ...ummm... 'invalidly'. It is such a temptation to toss the word around to mean 'anything I think is right or good'.
When you say "Valid means having merit compared to some sort of test,", it leaves the field wide open to ambiguity and equivocation as to WHAT test and who applies it to determine 'validity'.
We really need some very precise terms to use in certain discussions to avoid the "no it isn't-yes it is" debates.
*I* need a way to express "Hey.. even IF what you believe were to be 'true', your argument form is flawed."

Singing horses may soon be 'valid' if this continues.... ;>)