If Shakespeare was here today I am sure he would be asking the question "To lock or not to lock my luggage when I check it in?" This is a hotly debated topic.
Some people say that by locking your luggage you are given the impression that you have valuables to steal so you make your luggage a more attractive target. Even if you lock your luggage a thief can still find ways around the lock by forcing the zipper or hinges.
Other people argue that the level of petty theft is so high that by not locking your luggage securely with a TSA Approved padlock that you are asking to be a victim. There isn't much data available on baggage theft as it is lumped together with lost luggage. Advocates of locking luggage also believe in the outrunning the bear scenario which makes a lot of sense. To survive a bear a chase by a bear you don't necesarily have to outrun the bear you just need to be able to outrun the slowest person. If you lock your luggage and the luggage next to yours isn't locked there is a strong arguerment that the opportunist thief will target the unlocked luggage.
Personally, I never used to lock my luggage until I had a basebal cap and a few other small items disappear and now I religiously lock my luggage with a TSA Approved padlock. The other benefit of using a travel lock to padlock your zippers is that it stops them being accidentally pulled apart on their journey to the underbelly of the plane.