Using the lock as an insurance policy is pointless, the thieves will often take the lock with them.

It's better to overlock your bike when it's at home or work (unless you work in a secured area, then light locking may suffice), and to slightly overlock when travelling around. I often use the insurance the lock carries as a gague as to which one ot get for that bike. At the least, it's a rough estimate, but it's better than just saying "chain/big chain/u-bolt....hmm...huuuuu....ohhhhh....hmmmmm...beats me I'll buy the u-bolt"