Here's the answer:
Tight houses are good because they lower energy bills and increase comfort.
In doing so, they reduce the amount of work that an HVAC system must do, so the system can be smaller.
This can save you money up front and improve your finished product in the form of better comfort and energy efficiency for your customers.
(And here comes the big but ...)
But well-insulated and tighter buildings also behave differently from their leaky forefathers. Controlled ventilation is critical on the inside to keep a steady flow of fresh air in the house.This is a good thing. In the old days, we opened windows. You can still do this in a tight house, it's just that you won't need to.
One more big but:
If the inside of a tightly-built well-insulated wall gets wet, it is much more difficult for it to dry than a wall with no insulation.