Choosing bike features - gear shifters

When you are looking to buy a bicycle, you may want to be choosy about the type of shifter it has.

I have a bike that shifts gears when you twist part of the handlebar grips, as shown below. I can steer, brake, and shift at the same time.

Twisting grip shifter on a commuter bicycle

This is a nice ability to have when coming to a stop after speeding downhill or when riding on rough surfaces that make me afraid to let go of the handlebars.

Almost as good, in my opinion, are finger/thumb lever shifters, shown here:

Index thumb shifter on a mountain bike

I'm not fond of bikes with shifters that cannot be operated while grabbing the handlebar. For example, my old road bike has the shifters placed on the down tube.

Shifters on the down tube of a road bike

On rough pavement I want to hang onto the handlebars; in that situation I sometimes avoid shifting, or skip gears (e.g. shift from 1st to 3rd or 4th) so I have to shift less often. Also, if I'm stopping after a downhill, I have to reach a near-stop early, let go of the rear brake lever, pedal so I can shift down to the lowest gear, then stop myself.

How many different gears/speeds does a bike need? It's nice to have a lot to choose from, but I'm guessing that many people could be satisfied with six or so. The gearing should meet these needs: