You are confusing yourself a bit on this.
The other persons property damage coverage pays for your car (if he is at fault). His Liability coverage pays for you, if you are injured.
Your underinsured BI coverage extends their liability coverage, in case there is a gap.
His property damage coverage (not his collision or comp coverage) pays for your car. You are right, everyone should carry enough coverage to pay for the damage they cause, but that's not required, so a lot don't.
Your own collision coverage extends the other persons property damage coverage. If you have collision coverage on your car, your car will get fixed (or totalled if the damage is enough). This isn't replaced by the very inexpensive UMPD coverage.
Dan
The other persons property damage coverage pays for your car (if he is at fault). His Liability coverage pays for you, if you are injured.
Your underinsured BI coverage extends their liability coverage, in case there is a gap.
His property damage coverage (not his collision or comp coverage) pays for your car. You are right, everyone should carry enough coverage to pay for the damage they cause, but that's not required, so a lot don't.
Your own collision coverage extends the other persons property damage coverage. If you have collision coverage on your car, your car will get fixed (or totalled if the damage is enough). This isn't replaced by the very inexpensive UMPD coverage.
Dan