I agree. Just because you have gears doesn't mean that you have to use them. Couldn't the purists just stay in one gear but still have the option to change if they want to. I don't understand the current fixation (no pun intended) with sigle speed and fixed wheel bikes.
Yes you could do as you suggest. Or you could only use every other gear, or only gears with an odd number of teeth... You can do what you want. There are no rules other than the ones I make up arbitrarily.
But that's not really the point. If you like old cars, you can get 'close' to the sensation in a modern machine by switching off the a/c and double-declutching when you change gear.... but it isn't really anything like it.
You can chop your own wood for a solid-fuel stove in the sitting room, but it's not the same as life in the Edwardian period.
I was a fixed-gear sceptic until I built one and tried it. It's not a fixation (although I like the pun). It's not a cult or a new religion. It is great fun. Pure, yummy, distilled fun. In spades.
I now have a freewheel sprocket on the other side of my fixed wheel, and that's fun too... Jury's out as it's a new mod, but I like it so far.
Neither gives anything like the sensation of riding a geared bike in only one gear - which I tried when deciding on sprocket sizes for the fixopholous.
There really is a joy to riding fixed-gear that other bicycle types can approach but not replicate. Last year I did a 125-mile charity run with my daughter, who was riding with gears. We clipped along at a nice 16.5 mph average until we hit outer London (Gerrards Cross) and were slowed by Saturday shoppers to our destination (Trafalgar Square). She took the Mickey out of my 'Zenlike communion with the machine' all the way to London, but it really was a hoot. I have a geared bike too (more than one) and I love to click up and down the available ratios... but fixed gear is like the perfect Gin & Tonic on the perfect evening in perfect company - with a Bread & Butter Pudding for afters.
As to Drago's comment about having 30 gears... Is this chap suggesting a granny gear on a roadbike? I do hope not. Some things are just not done, old boy.
If it's on an MTB, I accept it... but only just. Roadbike? No. One of my rules and they are not up for discussion.