You don't always need a C wire with Ecobee
On MGG #766 they were talking about needing power from your furnace to run the Ecobee Thermostat. The Ecobee comes with the PEK, which can be used in installs that don't have a C wire.
If you need to run two stage heat, AC and Fan you'll need 6 wires in a traditional Gas, forced air set up. The Ecobee will let you run that same equipment with 5 wires using the included PEK.