Why run at a 1ohm load?

I dunno about that bit about the "efficiency decreases and can cause more harm to the subs".  If the amp is driven into clipping for extended periods of time then the subs can be harmed, but I don't see how decreasing the impedence would make the amp easier to clip.  ANYWAYS...  in most situations you need to worry more about pushing the amp too hard, than about hurting the subs.

When you decrease the impedence, you increase the load on the amplifier--basically your subs will try to suck out more power from the amp than it can reliably dish out.  As Dan MC pointed out, the amps will end up getting very very hot and eventually shut down.  Theoretically the built-in thermal protection circuits are supposed to prevent the amp from being permanently damaged, but I've always been dubious of the value of such protection circuits.  Better to just run the amp the way it was intended to be run.  Yes sometimes you can use cooling fans to keep an amp that was not made to run at increased loads, stable and not get damaged.  But this doesn't always work unfortunately.  Even among 1-ohm stable amps, there are some that dissipate heat better than others, so it might be something to think for these kinds of amps.

Some amps are 1 ohm stable and are made to run at increased loads, but these are usually only the higher end models.  It should be noted that most amplifiers out there can only handle a 4-ohm bridged or 2 ohm stereo load. 

Some subs have two 2-ohm voice coils and thus when they're wired in parallel they present a 1-ohm load to the amp.  But these subs are also the higher end models.

Finally another thing to note is that while an amp may be stable down to 1 ohm, some of them don't put out any increase in power below 2 ohms.  So for these amps, being 1-ohm stable is really just a system flexibility thing (i.e. it can support powering more subs at once, but it will  still only be able to use the same amount of total system power).