my house rules...
this was an issue in our group a while back. I'm the DM and IMO while you can gain some extra power into an attack using both hands on a double weapon AS a double weapon, you sacrifice some of your swing distance i.e. there is a bit less momentum buildup. Try taking a baseball bat and swing it holding onto the grip, then put your hands nearer the end and strike with one end. Theres no doubt the damage isn't really on par with a full swing. As such, I see no realistic reason for allowing the 1.5X strength bonus. OTOH, I believe the offhand attack may be a bit more effective than a typical offhand attack, so I have allowed the full strength bonus on the off hand attack. So, in my game, there IS a slight advantage to having a double weapon i.e. you get your full strength bonus on both attacks.
One last point...when he said you'd be better using a regular 2 handed weapon if you were always using the double weapon AS a simple 2 handed weapon, there are SEVERAL reasons that this would be true.
1. its cheaper
2. they typically do more damage
3. as a DM, if someone were doing that regularly with a double weapon and did a critical fumble, I'd say they're a LOT more likely to injure themselves.