Not to be too cynical, but the benefits to the clients in this are few and far between. Maybe somewhere there's a big data angle here to help the new company better compete with Google, but even the clients don't expect much to change. Anheuser-Bush InvBev VP of Marketing, Paul Chibe was quoted in Ad Age as saying, "It doesn't change anything."
So this larger holding company will gain efficiencies in back room operations, be able to consolidate some real estate, and probably jettison a few thousand newly redundant staff members making it less costly to produce the same amount of work. All reasonable business moves.
I can guarantee you, however, the monetary gains from increased efficiency won't result in lower rates charged to clients.
I'd love to be Maurice Levy's real estate agent today...