I can see both arguments here, and while I don't want to continue it after DXV has made it fairly final, I still think the while in play argument is interesting.
Fundamentally the question is "what would happen if Highway (say) had its while in play effect above the line?"
I would argue that by Throning it, this happens.
1) I play Throne Room
2) I choose to play Highway twice. It enters play.
3) I get +1 Card +1 Action, and Highway is in play, so all cards decrease in cost by 1.
4) Throne Room continues, I must play Highway again.
5) I pick up Highway (OK noone ever actually does this, but it's how I've always imagined Throne Room is kinda meant to work - it's how Androminion does it for instance), in order to play it again.
5a) Highway has left play; all cards increase in cost by 1.
6) I play Highway again. It reenters play.
7) +1 Card +1 Action, Highway is in play, all cards decrease in cost by 1.
Overall effect: +2 Cards +2 Actions, all cards decreased in cost by 1.
I guess the argument is whether step 5 happens. If the Highway never leaves play in between playings of it, then the first effect never gets cancelled out, so the effects would stack without a dividing line.
I'm sure someone will tell me whether I'm right to think that cards leave play for an instant in between each playing by Throne Room and variants.