Because we are putting in a marble floor, we were concerned that the floor will be cold on our bare feet. Half the year, this will be a good thing, but the other half of the year, it will make it uncomfortable. We don't want to put a heater under the marble, because this room does get a lot of sun since it faces south, and it warms up quickly. The real concern is nights and first thing in the morning.
Taking out the linoleum and underlayment, we can see sun through the slats of the subfloor. We have a lot of ventilation under the house, which is necessary in this climate. But - that means there are cool breezes blowing under the house when it's cold out - so we knew we wanted to insulate the floor. Besides the open crawl space breezes, this house doesn't have central heat, and this space is the furthest from the floor heater.
I don't think it's just my imagination, but the room feels so much warmer even tonight! Before he puts the backerboard underlayment, Dale is going to get the electrical, gas and plumbing relocated while he can easily access the space. We're pretty happy with this, and we'll be insulating under some of the other areas in the house that are cold (in particular, our bedroom!).