Sure, cutting out car commuting is nice.
However...
Days when it is pissing down with rain, it is dangerous and a rather unpleasant experience for me to ride in the rain. I drive my car instead. Sure, it costs me $3-4 to do so each day, but sometimes I wake up and just don't feel like riding my bike because the weather is shocking, I'm going somewhere else after work or lunchtime where I require a car, etc.
If you aren't saving money and are spending $100 more per month just to be able to walk/bike to work, not worth it. That $1200 per year you would invest or do something meaningful with it.
Cutting commuting costs is nice, but just not 100% practical for everyone. I'm more conscious every time I use the car now, but I'm still reliant on having a car. I do enjoy riding my bike to work though, it is good fun and after 9-12 months or riding to work 3-4 times per week I made my money back on the bike purchase which is pretty cool.