Thank you for being understanding and painting both sides of the "argument." It's greatly appreciated. As I said, I'm going to play this out once to see what I get and I can't stand having to justify that decision.
Anyways, another way of looking at it is that we don't now if Cullen is really over it. We see him relaxed and feeling better on the battlements, but we also know that he still gets headaches (war room banter) and has nightmares (probably for the rest of his life). Cole says that templars are permanently changed. Who knows if Cullen will have any more adverse effects down the road...the withdrawal may catch up with him later...dlc?
I hate to be the Eeyore of the discussion, but it's a definite possibility.
That being said, yes, 99% of the time I do tell him stay off it.
It's not even about "justify" or not... Cullen's taking a very great risk on staying Cold Turkey out of Lyrium. To be fair, half the time when I first knew about that & told him to stay off Lyrium, I expected something bad will happen. Still happy that finally he can be happy & well at the end of DA:I without major consequences. 
And to be fair, on RP-ing you don't know whether he'll be better or worse. It's by meta-gaming that we know that game mechanics allow him to have incredibly thick plot armor.
In any case, meta-gaming, even if you choose to keep him on Lyrium the convo can go on a very gentle & supportive way for him. It's not always in a negative tone. 
EDIT: Cullen back to Lyrium or stay off lyrium, the choice had never been black and white for me... Both are grey.