The beauty of using points or EMs on computer systems is that the measure is abstracted from the constraints of the hardware. So a point isn't just 1/72" of an inch, period. It is a means of insuring consistency across myriad platforms.

For the designer who wants consistent presentation across a diverse range of devices the use of traditional font measures provides a far more reliable mechanism than using pixels or percentages in underlying style sheets.

At the very least it prevents us from having to rely on Microsoft, Webkit, or Gecko to do math and get it right.