Where I live, bike theft is a common place, sadly. My bike is my pride and joy so I make sure I have my eyes (my friend’s eyes) or carry a lock, at all times. Many probably feel the same.
So the short answer, it depends. The question to some may seem like an obvious answer. To those who come from a casual riding background with friends, stopping for a snack-break, running errands, and leaving your bike unattended, a bike lock is a must. Then there are the cyclists who are kitted up (in their cycling attire), riding shiny bikes, and only carrying water and a repair kit, with no bike lock. What? The bike is so expensive, why would they leave their home without one?
Here are some schools of thought to help you decide when to consider bringing a bike lock.
What is your goal for the ride?
Are you riding by yourself for a quick workout and then going back home? No lock. If you spot a cyclist with very minimal items on them, most often, they intend on being on the bike majority of the time or taking a brief stop with visibility to their bike.
Do you plan on running errands, use it to commute from Point A -> B where you need to step away for a given amount of time? Yes lock.
Are you riding with a group?
Biking far? Typically no. There’s an implicit code of conduct to watch each others bikes as someone steps away. If they are new friends, do always ask and offer to watch.
Taking a pitstop at a cafe? Optional lock. Take into account if the cafe and area is bike-friendly. For example, does it have outdoor seating where you can see your bike and it’s relatively safe to leave alone? Then you probably don’t need a lock. Even better if you can place your bike within arms reach there.
Not sure if the group ride will require one? Ask the ride leader if it’s recommended.
Are you riding by yourself and plan on..
Taking a long break and leaving your bike where it is out of your field of view? Yes lock.
Stopping by a cafe where you CAN see your bike when you order something? Optional lock. Do what makes you most comfortable when you leave your bike unattended at the destination.
Note: This is more of a concern in areas with high theft rates. Keep in mind the amount of foot and car traffic when you are taking a pitstop. Thinking about this requires some planning prior to a ride so you can stop with certainty and know what to expect when you plop your bike at an unknown location. Use your best judgement and research here.
Depending on your style, a bike lock may be more or less of a necessity on your rides. If you decide to carry a lock all the time, hey, that’s peace of mind, an extra workout, and more power to ya! I’m big on planning and generally keep the above thoughts in mind before a ride. In the end, do what makes you feel the most comfortable and trust your gut wherever you decide to park your bike.