Strategic acquisition or merger, what value?

Large external growth deals are back: AB InBev’s public bid to take-over SABMiller in the beer industry, the acquisition of EMC by Dell in technology, the merger of Holcim and Lafarge in
cement, … These major acquisitions and mergers are often criticised for both the high price and the low probability of success of their synergies. Why pursue such strategies? What is their value?