How paranoid are you when it comes to leaving your bikes locked up in town/outside your work etc?
I generally use two bike locks if I plan to leave the bike for a long while. I have a thick, heavy duty one in my backpack at all times and a thin, light one wrapped around my seat post.
This was mainly due to having quick release wheels on my bike and this way I could put a lock around the front wheel, the frame and whatever I was locking up to and then do the same with the back wheel.
If I am only leaving the bike for a short time, such as running into a shop to quickly pick up something I will generally just use the small one I keep around the seat post.
I've never understood how so many people can feel safe just leaving there bike unlocked while they pop into a shop. All it takes is one second and your bike is long gone.