The article below lists seven reasons why email newsletters work. However, within a business-to-business context, the first one, 'Different People, Different Channels', is the real killer. 

Frequently, the people we're trying to reach with B2B content are busy professionals and, whilst many of them are on Twitter and other networks, the work-horse channel of business communication remains email. So having a relevant, timely communication that sits within this work context makes perfect sense. 

Increasingly also, email newsletters can be an effective proxy for a dedicated mobile content marketing strategy.