I see this kind of statement all the time. While it is technically true, it's not practically true. Think of it more as "a 21-speed has 3 7 gear ranges" and you're closer to the purpose of the system -- shifting the front ring is a very coarse adjustment to get you into the gear range you want to use, and then the rear cogs are the fine tuning. I shift the front ring far less frequently than the rear cogs.

I spend most of the riding season in the big ring (flats where I live). When it gets windy or I'm working on my base, I'll leave it in the small ring.