I guess its easier/cheaper .... maybe a selling point as don't have to modify the fitting?

Its also going to be wasting power. The ballast will still have resistance (as well as alot of reactance) so there will be an I2R power loss. Only a few watts but its still makes the whole fitting less efficient.

There is a safety reason for doing so. If someone mistakenly puts a standard tube in the fitting, and the tube struck, it would then explode due to the current flow. If the choke is left in circuit, it will limit the current and the worst it can do is stay illuminated. 

Regards,

Alan. 

Thankyou for your replys I'm familiar with the way a normal flu lamp works and what the choke does. I've always had an interest in lighting. I think I can say to the person who asked me about chokes in LED conversions that it makes no difference in also I will point out the safety I p,ications if a real tube is put back in with no choke. Myshack/workshop is lit by 2x36 watt magnetic ballast tubes plus one HF ballast tube wattage unknown  I've noticed in this cold weather the mag ballast ones flicker when very cold but the HF one although at the same temperature just appears dim with a noticeably dimmer patch about a third of the way along. I'm not concerned about it its just an observation