Bicycle locks are very necessary if you are going to leave your bike unattended where it is at risk. You have to try to think like a thief and use a lock that will be tough enough to do the job.
You not only have an investment in your bike but it's a heart-breaker to get your bike set up just the way that you want it only to have it stolen.
Common sense can dictate your locking choices. Locking your bike in a well lit rural area is much different than on the street in a high crime, inner city, area.
But, if you have a big investment in your bike don't assume that it is safe anywhere and get the best lock even if it is a bit heavy
Basic U lock
In some cases it may be advisable to use a large U shaped bicycle locks made from 1/2" diameter hardened steel. They are inexpensive and do the job nicely but can be a bit heavy to transport conveniently. Check the depth of the lock to make sure it will suit your bike and how you want to lock it up.
Always make sure that whatever you lock your bike to is at least as strong as the lock.
Another good choice is a hardened steel cable. The lock shown here is a high security armor plated stainless steel locking cable (6 feet long x 26mm diameter) for maximum protection and security.
On the plus side, you can transport it wrapped around your seat post so it doesn't take up room in your bag but it's not as theft proof as a U lock.
The more tools that a thief needs to steal your bike the more likely he is to move on to weaker prey.