How important is the Centre Speaker ?

Something like 60% of voices come through the center, so to me very important. You could make your receiver shift this audio to the fronts, but I personally don't like that.

Very important. I think people should use the same speaker that they are using for their mains for the center channel.

Not important at all to me. I've tried many different center channel speakers, and found no center channel to be the best option. I prefer the 'phantom' center channel. I believe if your front speakers image properly, the center speaker only messes things up. That's my $.02.