There are actually some very good reasons to include the attic in the heated space (e.g. if your hvac conduits are inside your conditioned space rather than running through a blistering hot or freezing cold attic, you can prevent fairly significant losses and actually save money even though you're heating a larger space), but you have to make sure your attic, roof, and HVAC systems are set up for it. You shouldn't just slap insulation on the bottom of the roof deck and assume it will work, it may not and it also has the possibility of creating things like moisture damage problems in your roof depending on your climate.