The benefits to the phone, you already have it and keeping it charged and in general not forgetting it at home are more or less taken care of.

If I were considering options for additional units to purchase for determining occupancy (perhaps the kids do not have cellphones yet) then a small zigbee type mesh sensor would be ideal. This type of solution would give many options for those with zone based heating and cooling and would lend itself to even more pretty visualizations.

Edit: Getting a post right the first time from a phone is harder than it should be.