Hey Jeff,
I have a couple thoughts on this. The first is that lets be honest, there aren't many Christian scholars they can point to, in order to back up their viewpoints on certain matters. Modern day for sure. The scholarly community is very close knit, and in many ways like the medical community. You have to accept certain things to even be at the table to be taken seriously. I am not saying its right or wrong, just that it is true.
Next, I see what you are saying with the old light new light thing. At the heart of your question, am I understanding you are asking if it is ethical to accept the writings of someone to support your viewpoint on doctrine, even if that person at their core didn't believe int he doctrine you did?
I honestly don't have aproblem with that. The ECF's changed their minds all the time. I suppose much more interesting would be WHY they felt one way or another, not just that they said it.
Now that being said, if the intent with the quote is to DECEIVE someone into thinking Tertullian didn't know what he believed, then that would certainly be wrong no matter what. Didn't Tertullian coin the term Trinity?