Gears

I've been having trouble with my gears. I ride a ten year-old 21-speed mountain bike, and despite the fact that I have a brand-new drive train on it, my gears are acting up.
When I'm in my high gear at the front, I sometimes have a devil of a time finding the low gears in the back. And it's got me thinking: do I really need 21 gears?
Honestly, I never use most of them. I rarely leave my top gear of the three in the front, and only use a few of the seven in the rear. Sure, if I was cross-country riding across lots of mountains, some low gears would come in handy, but as a commuter and trail rider, most of them are wasted.
I figure that I only need about five gears:
- go like hell;
- medium cruising;
- cruising in traffic;
- little hill;
- big hill.
Give me those five gears, and I'll be happy.