Since content consuming is radically shifting towards mobile devices, your question sounds a bit like "why is it necessary for marketing to use platforms where people are?"
The only case ignoring mobile can be justified is when you have a clear target market which is not on mobile, but can be efficiently reached through traditional online channels.
Online marketing can still survive even if it ins't mobile friendly, but it may not be the best solution.
In 2010 Microsoft Tag did an interesting study and projected that by 2014, mobile internet should take over desktop internet usage. Because of the increasing use of mobiles, its versatility is always growing where one half of all local searches are performed on mobile devices, etc...
Because of these reasons I think it is in any companies best interest to provide mobile-friendly online marketing. Why not market to everyone instead of excluding a large population of people?
Hope this helps - cheers!
Smartphones and tablet PCs are increasing their popularity and many people preferably use mobile gadgets for browsing. Online marketing still can exist without becoming mobile, but it will be less efficiently.