Honestly I think it's because of the design. They didn't want to break up the silhouette of the iPhone so they recessed it.
Actually I think its purpose is twofold...
There's the aesthetics, you're right on about that... Apple is very particular about industrial design and a surface-mounted port would look awful.
There's also damage protection... I was thinking about this the other day, and it occurred to me, as it probably has occurred to a number of people by now, the shoulders around the jack keep the plastic sheath firmly in place.
One of the biggest complaints I have about the older non-magsafe power adapters is that yanking the cord will invariably loosen and eventually break the $80 power cable right at the point where the plastic shielding ends and the metal shielding begins.
This would be the same case if you snagged your earphone on a surface-mounted jack... it could end up breaking the metal tip off, which would get stuck in the phone, and then you're screwed out of $30 for the headphones and god knows what for a repair to disassemble the entire phone to get the metal out... because if you've ever seen headphone jacks they're generally soldered to the board and you have to remove plenty of internal components in a case as tightly packed as the iPhone (or the MacBook for that matter) just to get at it.
I nearly lost the tip of my power cable (the jack of which is also soldered to the motherboard) and they managed to get it out in the store, but if the Mac Geniuses couldn't have, it would have been around $200 labor.
I imagine with the iPhone being less "friendly" to disassemble (lacking any visible screws, etc. on the outside casing) the labor to fix such a disaster would be just as bad... if not worse, depending on whether or not it damages the internal jack... and that doesn't begin to compensate you for time and productivity lost if you have to be sans iPhone for a couple weeks due to a friggin broken stereo miniplug.
So, IMO, Apple was actually pretty damned smart to design it this way.
The inconvenience of getting an extender/adapter is far less than the inconvenience of xbreaking your brand spanking new iPhone.