Yes you are, unless the terrain around you is very flat, with an occasional hill. It would be very difficult to keep a steady cadence by just using the front derailleur. If anything, it would be more common to just use the rear derailleur. You have a lot better chance of keeping a steady cadence when you have 6 (or 7, 8 or 9) gears to choose from than 2.

I don't know your age, but as you get older, mashing is very tough on your knees. By using the full compliment of gears on your bike, your knees do not take the beating.