Several hundred people sitting all over a gigantic room, not a few people in a loungeroom, sitting on a sofa staring at a flat panel 10ft away.
Lots of truth in that.
To be a little more focussed, the 'correct' seating distance for stereo hi-hi is generally thought to be based on an equilateral triangle. As such, the angle between you and the 2 speakers will be c.60 degrees (30 degrees each side, from a line drawn between you and midway between the speakers).
In a cinema, you're going to get a lot of people sitting towards the front who have a far wider angle than that, which is the main reason you need a centre. If you placed dialogue which you wanted to come from the centre in a stereo pair, you'd feed it equally to both channels. If you were sat too close to (for example) the left side, it'd sound like it was coming from the left, rather than the centre.
For home cinema, dolby suggest 22-30 degrees, so similar to stereo hi-fi.