LadbrokesLadbrokes is a prime example of what you can do when you use subdomains effectively. With massive presences in most of the niches, they’re clearly doing something right. Now I’ve always advocated the use of subdomains over subfolders in many cases, particularly languages (except for China, Russia, Korea and any other market where Google doesn’t dominate in which case you NEED a different site. Remember optimization is different there).
They aren’t the only people to use the authority of a strong domain to pump juice through the juice Paddy Power use the same trick. It is a good way to lower marketing costs than having different domains for each niche if you plan to attack a broad range of markets. This also gives a coherent brand image, however some would argue that this is only applicable to the really big players. But, perhaps it is something to keep in mind when starting a new domain.