The "pedal under the arch" idea might be more secure, but to me it feels *totally* wrong, whether on flats or clips. A lot of my suspension comes from ankle/calf movement,and I tend to flick the bike around (especially in clips) purely by ankle movemnet and if I put my foot further forward on the pedal I lose that - the bike feels completely dead and I'm unable to get near as much control or grip - all impacts just go straight up my legs and I'm far more likely to bounce off the pedal.
Do you ride with your legs dead straight and knees locked Jon? I agree that you can use your ankles a lot, and even with the cleats back as far as they go in the slots I still use my ankles a lot. The difference in travel you can get with your ankle might be slightly reduced, but I find the stability more than makes up for this.
More than this, with the cleat right forward you are relying on the smaller muscles of the shin and calf to control the bike and the body weight. Once you move the cleat further back you start engaging the glutes and the quads which are way more powerful muscles, giving you more control and less fatigue...
Pedaling with my arch, again, just feels so wrong - no power, stiff legged and dead. I have to faff about moving my feet to the right place (ball of foot over axle) before I can concentrate on anything else.
If you are used to using a specific set of muscles to pedal, and these muscles are the ones you utilise when the ball of your foot is over the pedal then trying to pedal with muscle groups you are unused to will feel a bit dead and less powerful. When I changed my cleats from right forward to right back it took about a month for my legs to feel as strong, but now I am just as fast and strong up the hill, and I have more control and am going faster down the hill.
You have to do what works for you... There are a lot of people out there who will tell you what is right and wrong, but there are not often a lot of them basing what they tell you on actual evidence. This cleats back thing works well for me, and for other people I know... but everyone has to make up their own minds.