Lego's argument always struck me as absurd. Imagine Ford doing the same thing: "Please refer to our vehicles as 'Ford vehicles' or 'cars', and not 'Fords'. By doing so..."
I think the temptation to start a car company that markets "Ford" cars is probably less than the temptation to put on your box of bricks that they are compatible with Lego bricks.
If the company doesn't take reasonable actions to protect the mark, they lose control of it.
The only risk I can see is becoming a generic term for "construction bricks". I don't see how me calling my genuine Lego bricks Legos contribute to that.
If the company doesn't take reasonable actions to protect the mark, they lose control of it.