USB carries both power and data, while MagSafe carries only power. This doesn't sound like a problem until you realize that the USB pins have to be unplugged in a strict order: data before power. This is enforced in the connector with varying pin lengths. With MagSafe, there is no guarantee of connection order.

That seems like an easily surmounted problem to me. A MacSafe-like connector could probably easily accomplish the same if there was a need for it.

variable pin length in the magsafe

Simple: it carries data. If the power cord on your laptop is yanked off for a minute it's usually not a big deal. If you disconnect the iOS device cable you could interrupt a data transfer. There is a higher risk to disconnection.

It is USB, and the host could lose power at any time (it doesn't have to be a laptop). Communication interruption is a likely case they should plan for.

A more probable explanation is that even with a magnet the devices are light enough that a yank on the cable would send the device flying before the magnet connection came loose.

I'm sure they do plan for interruption, but with magsafe you invite it, in a sense. Especially on a portable device.

You're probably right that the physics of the magnet don't make it as useful. That makes me wonder how thoroughly Apple has investigated it.

Those two cables do use the same design. I believe that the magsafe cables are a bit more durable and I think that's mostly because those cables have (necessarily) more isolating rubber.

The MagSafe design has four fairly large pins. The iPod Dock Connector has about forty tiny ones. It wouldn't work well mechanically.

A USB port has 4 fairly large pins...

Why couldn't they just only use the four pins included with USB? (although that would mean an end to the standardized dock connector)

Not sure if a strong magnet in your pocket would be good for transit cards or other mag stripe things.