If you're driving stick, then you need the left foot for the clutch, so it won't be free for the brake. Left foot braking is a habit that translates poorly to manual transmission. If you only ever drive an auto, it does not matter. Since you're going to stall out a lot or possibly not even get out of the driveway if you don't unlearn this habit when driving stick, it is a problem that will correct itself if you do switch to manual.
The only other problem is that some people have a tendency to drive with their foot on the brake pedal all the time. If you do this, even if you can't feel it, you are probably wearing out your brakes AND reducing your fuel economy, and you're probably reducing the effectiveness of your brakes when you actually stop.
If neither of these apply to you, it is not a problem.