Separately, the second-ranked Hakuhodo (let's forget about Daiko and Yomiko for now) couldn't muster enough of a challenge to get Dentsu out of bed. But through their latest union, the trio will control 18 per cent of the ad market. Although, their combined share still places them some distance behind Dentsu's 25 per cent, they're now a far more credible challenge than Hakuhodo's pre-alliance 12 per cent.
As the country gears for its fourth recession in a decade and major advertisers prepare to pare down marketing budgets once again, the cost saving imperative is clearly driving the rush to the altar. But it's also a defensive strategy.
The agencies' senior management have reached an age when they need an exit strategy, and the partners must take Dentsu down a notch or two because future growth will not happen organically but will have to come at the expense of rival agencies.
Sizeable though it is, Dentsu is already feeling the heat from the shaky economy. Last month, Dentsu reported that its first half current profits had tumbled by a quarter as the almost US$50 billion advertising market reeled from the most dramatic fall in 17 years. If the heat has been turned up for Dentsu, agencies further down the Top Ten rung who are outside the Dentsu or Hakuhodo fold will find themselves especially vulnerable.
That includes the third-ranked Asatsu DK, which has an alliance with WPP, Tokyu and Asahi.
Equally, the massive buying clout of the two blocs and Japan's complex media environment - where Dentsu has a stake in several media companies - will only make trading conditions more difficult for media independents.
The planning skill that multinational media agencies bring to the table is of course important, but it's only relevant if they also have the buying clout to sell with it.