But why should I not drive in electric ????

Ok, for the 1000th time: Driving in electric only is NOT desirable if your goal is to have the best fuel economy and emissions possible !

Why would this be? Isn't this the objective when buying a hybrid vehicle to begin with?
Answer: NO. The main reason why it is not a good idea is because the energy contained in the battery pack was acquired as a result of a slow down (braking) or through a forced regeneration (gas engine charging the battery pack).
The means to greater fuel economy is steady and lower speeds and a desire to conserve momentum (remember your high school physics?). Furthermore, the energy you place into the battery pack is far greater than what you'll get out. This happens because of the energy conversion losses and the more exchanges are there between the battery pack and its upstream and downstream systems the greater the losses.
For the more technical, these losses manifest themselves in the form of heat generation, internal resistances, parasitic interferences and frictional losses.

With this said, hybrids are still more efficient than regularly powered vehicles because they recover energy whereas the others would simply waste it all away in the form heat and friction.

The other point I want to touch on:
A Prius is good for 2.9-3.9L/100km in city duty. On the highway, you should be able to get at least 4.4 l/100km.
A Civic hybrid is good for 3.6-4.1 L/100km on the highway. You can also get this performance in city driving but you have to throw the entire hypermiling book at it (forget eco-driving).

Remember, speeding (traveling at or above the posted speed limit) is a sure way waste fuel and traveling at more than 100km/h (62 MPH) should be avoided.

If you own any of these cars and you are are not getting the mileage I mentioned then it is time to learn a few more tricks about your car, but the simplest you can learn is still to stay off the battery pack. I cannot stress this enough.