My honda accord has the immobilizer and factory alarm
on it, but the factory alarm didnt have a shock sensor
or flashing light.

Consequently I had an aftermarket Clifford alarm installed
with shock sensor.

Worthless you suppose??
Well I park my car with gear (eye candy to young thieves..'surboards&wetsuit,etc') in it quite a bit, and the alarm , its flashing lights,
the clifford stickers, and an alarm that will go off when
someone bodyslams themselves into my car discourages
young 'smash and grab' thieves, especially the ones
who want to 'smash and grab' in quiet solitude.