FWIW - I asked something similar a while back, so you'll have some additional responses:
Thread: When do you drop collision?
To me, it's a question of how much cost do you want to endure every 6 months versus what cost can you endure if you need a car tomorrow and what needs do you have for a car when it's "tomorrow". Next question is what do I think my odds are of needing coverage.
We have 3 vehicles:
'13 Cruze with a loan so no choice there
'06 Cobalt - no comp or collision
'99 Saturn - no comp or collision
If the Saturn or Cobalt were gone tomorrow, it's not a real pain point in the short term since we only really need 2 vehicles. Additionally I'd think the Saturn is worth close to $0, so the only thing I'd be getting coverage for is glass. Comp with glass is $80/year and collision is $90/year both with $1000 deductible $0 glass).
Cobalt is worth...? Not sure what it would be valued at, but it would not be terrible to go out and replace it tomorrow. Comp with glass $140/year collision $240/year, both $1000 deductible.
For us, we do fall at the higher end of mileage a year 40K annual miles 2 people to and from work, congested area so that does create some concern for me, but not worth the premium if we need to replace. Liability side is a different story.