The hairdryer as heat-gun method was first shown to me by an incredibly gifted Electrical Engineer (from China) who used this method to fix dry joints on PCBs (including consumer mainboards).
It was astounding the first time I saw a dead MB revived with nothing more than a bathroom accessory!
You're almost certainly right, I've not had success with it myself, and I originally thought the same, but I've heard anecdotal evidence from others that it's worked for them, and there are a few mentions of it online, including a video at http://www.anandtech.com/show/6840/hardware-tricks-can-you-f... , but I've also found mentions that the likely cause of the hair dryer trick is old capacitors being temporarily "revived" by the heat (http://www.instructables.com/id/Fix-a-dead-power-supply-usin...). It seems like something is being at least temporarily repaired by the heat, but it's probably not anything to do with solder.
Hmm. I've got one here I use for heat-shrink tuning I used masking tape on to lower airflow and boost temp. I'm not sure its hot enough to melt solder (probably not) but its great for heat-shrink. I suppose a YMMV is in order.
It was astounding the first time I saw a dead MB revived with nothing more than a bathroom accessory!