http://en.wikipedia.org/wiki/List_of_mergers_and_acquisitions_by_MicrosoftSo which of these really contributed to Microsoft's success?
Sandy Weill was a better example.
But M&A does result in synergies. If big bank A buys smaller bank B, you move all the accounts and transactions on to bank A's software, servers, databases and networks. You shut down a bunch of data centers, terminate a lot of software licenses, scrap a lot of servers and network gear and fire a whole lot of IT people. Same for other departments, especially HR, accounting, legal, etc. If the geographies overlapped, you can shut down a lot of branches and offices as well. Been there, done that.
It works similarly in many other businesses. The whole category of general adminstration, marketing and sales expenses can be trimmed a lot, and with all the rules and regs these days every big company has a whole bunch of people doing the same administrative tasks as every other big company. These can be consolidated.
Blue collar workers can't be consolidated as much. However, the acquiring company will feel free to go over all the products and services and terminate any line of business that isn't making money. The acquiring company will not be sentimental about the fact that some factory or line of business has been around since the beginning.