用图片背景代替文本时，为什么不推荐用 text-indent: -9999em？

在 Compass 看到 @mixin hide-text($direction) 的方法，默认情况不推荐使用负值的方式实现图片代替文本，这是为什么呢？

原文说明：
left is more robust, especially in older browsers. right seems have better runtime performance.
为什么用 right 性能会更好呢？

@mixin hide-text($direction: $hide-text-direction) {
  @if $direction == left {
    $approximate-em-value: 12px / 1em;
    $wider-than-any-screen: -9999em;
    text-indent: $wider-than-any-screen * $approximate-em-value;
    overflow: hidden;
    text-align: left;
  }
  @else {
    // slightly wider than the box prevents issues with inline-block elements
    text-indent: 110%;
    white-space: nowrap;
    overflow: hidden;
  }
  @include for-legacy-browsers((ie: "7"), $critical-usage-threshold) {
    //Text transform capitalize fixes text-replacement issue when used in a <button> element on ie7
    text-transform: capitalize;
  }
}