If a host can team up and offer multiple services, do it. Clients love it, because they only have one place to shop, and get to deal with known quantities. It brings the host more business, too.
But...for the sake of the industry and for the sake of the clients, clients should be able to easily transition to a different provider for any of the services without losing the others (whether by their choice or because the host stops offering the service for some reason). Set up your contracts so that you (and the clients) know who owns any designs, mock-ups, high-resolution or PSD graphics, et cetera. Make sure that all services work together seamlessly and are easy for you to maintain, but don't intertwine the services so tightly that the client can't switch away if they so choose. If a customer wants a different web designer (maybe they want to let their college-age relative, who's in graphic design, handle the site design as a way to build their portfolio) but isn't able to easily stop paying you for design services...you may lose their design business AND all other business as well.
People who advertise a service and then short on delivering said service are a constant fact of life...but they're irritating (to say the least.)