We did find (quite by accident) that Apple may have more reasons behind not installing Flash by default other than the stated reason of ensuring that users always have the most up-to-date version. Having Flash installed can cut battery runtime considerably—as much as 33 percent in our testing. With a handful of websites loaded in Safari, Flash-based ads kept the CPU running far more than seemed necessary, and the best time I recorded with Flash installed was just 4 hours. After deleting Flash, however, the MacBook Air ran for 6:02—with the exact same set of websites reloaded in Safari, and with static ads replacing the CPU-sucking Flash versions.
In case you need another reason, here it is: using FlashBlock and AdBlocker makes your battery last longer.