Unless things have changed with the Rainbow, the liner is hooked in place and only covers the roof/top of the tent. So, looking straight down on the tent, the liner is bow tie shaped just as the head room of the tent is (wide at bottom ends and narrowing to 18" or so at center top).
What is your previous experience?
Do you rely on double wall for warmth?
The Rainbow liner will help with condensation but the tent is not very warm in winter, of course. [I know you specified 3 season, so substitute 'winter' with any other time it is chilly]
What about wind...?