A typical or common retail amp (non big power competition amp) that is designed for 1-Ohm has a certain number of out devices and a power supply capable of the load.
If you buy an amplifier that provides 1000 watts at 1-Ohm, the manufacturer will provided enough output devices and a large enough power supply to safely handle this under typical daily use.
Most often, there are enough output devices to handle another 100-150 watts beyond the 1000 rated watts.
When you run the amp at .5-Ohms, you may be drawings as many as 300-600 extra watts through the output devices.
Over time, this will cause a failure.
If you need 1500 watts at 1-Ohm for daily operation, buy a 1500 watt amplifier.
The red line on your tachometer may be 6000 RPM but you wouldn't want to drive all day at 6000 RPM.
You probably wouldn't want to drive it at 4000 RPM either.
Yes, there is always box rise but every one listens to music for differant periods of time, volume levels as well as differant types of music.
Typical consumers listen to hip-hop, rap, big bass music.
Not all electrical systems can handle the abuse of high power - lower voltage conditions and this greatly effects the life time of the amp as well.
Even when a big current battery is used to replace the factory battery, most often there is not enough wire run back to the amp.
even a dB Drag Street A competitor will run as many as 8-12 strands of 1/0 cable.
Over kill for typical daily use but a lot of people don't realise the voltage loss from front to back when the lights are on, Heat/AC, wind shield wipers...
Any way...just some comments from an amplifier manufacture of 24 years and I'm sure others will have there own opinions.