A number of different points here (none of which I remember entirely):
Before the days of electronic starters the arc used to be struck using a high voltage spike, while this does require more energy than regular 240v, its not 6 hours worth (average fluorescent tube consumes around 50 Watts).
Fluorescent tubes used (I am talking 15 years ago now) not to take kindly to lots of switching cycles. What makes it more efficient is a combination of lower energy consumption and a much longer lifetime of the tube (10K hours v 1K hour) which offset the higher cost of the tube itself. I think that this 'it's better leaving the light on' may be based on this concept of shortening the lifetime of the bulb everytime you switch on and off.
Depending on where in the a/c cycle you switch things on and off, you can subject the equipment connected to large (x6 - x11) voltage spikes- which is perhaps a theoretical argument that you should never turn anything off.
<goes off to lie down as it's a long time since I had to think about stuff like this>
