It's accepted that solid-state drives offer faster performance overall. However, Microsoft's decision to make SSDs compulsory in most computers used by its staff was driven in part by a more specific metric: the tendency for hard drives, especially older models, to introduce much slower boot times, frustrating employees and reducing productivity.
Microsoft PFE Matthew Reynolds discussed the decision during a presentation on improving boot times for enterprise PCs at TechEd North America 2013. He showed a slide from a group of representative systems measuring their overall boot times (seen above)
Reynolds stressed that the numbers didn't form a critique of specific drive models, since the sample wasn't necessarily representative of market share. What stood out was that 5400rpm hard drives were far more likely to suffer from delayed boot times, followed by 7200rpm hard drives and SSD drives.
"As a result, you generally can't get a machine at Microsoft without an SSD, because we believe it affects productivity," Reynolds said. While there are obvious exceptions (OS builders need to test on a variety of hardware), it's a policy that would be welcomed by many workers, if not always by those paying the bills.