Even so, a company will want to avoid losing the trademark: https://en.wikipedia.org/wiki/Generic_trademark#Avoiding_gen... (Lego is one of the examples as a success story for this approach)

Lego the company was fiercely protective of their IP. They had a patent they regularly enforced. Once the patent expired they took MegaBlocks to court trying to claim the shape of their bricks was trademarked (they lost this... but took it all the way to the Canadian Supreme Court).

It's also a sign that you're becoming a commodity which is bad for business. You can Xerox your document on a Canon, but you can't Google something on Bing.

Here is a specific example of them requesting customers to help protect the brand by not calling them "legos".

http://english.stackexchange.com/a/47223