我曾对你提到的那个网友问题做了解答，也在回复一位老师的质疑时提到过than is/are necessary 的问题，但并没有展开分析。在此，我对你的问题稍作展开分析。

我以国外英文原文书刊的句子为例：

A. Moreover it employs more workers than are necessary.

这句话的直接比较对象为workers (复数名词），因此than 从句也使用了复数are necessary. 实际上为了便于掌握如何使用，我们可以暂且理解为than后省略those （workers），因为比较对象相同，所以要省略，避免重复。这种理解仅是权宜之计。因为从语法角度，英语一般不说workers are necessary. 但语言逻辑告诉我们这里省略了比较对象workers。

但本句使用单数than is necessary. 也是正确的。两种形式的使用频率其实差不多，than is necessary 略高些。人们通常解释为than it is necessary that it should employ them. 这种理解固然对，但是它的不足之处是无法解释复数 are necessary了，这样会使有些英语学习者误以为than are necessary 是错的。可实际上，这种用法，英语里很普遍。

因此通过这个例句，我们可以得出结论：

Moreover it employs more workers than are/is necessary. 都对。（这个后面再分析）

我再给你提供几个例句，供比较体会：

1. No more officials are employed than are necessary.

2. Repair actions will be taken on more failed parts than are necessary.

3. I shall refrain from burdening you with any more figures than are necessary.

以上这几个句子，改为than is necessary 也是对的。

 请看另一使用than is necessary的例句：

B. Federal school aid equal to 5-10 percent of school costs is far less than is necessary.

本句中aid (单数）是直接比较对象，than后要用单数is necessary. 注意：这里不能用复数are，因为和比较对象的单复数形式不一致。

 请再体会几个例句：

1. The very wording of the exemption thus indicates that it extends further than is necessary.

2. Courts should refrain from invalidating more of a statute than is necessary.

3. I do not want to take any more of your time than is necessary. 

这些例句一一般也都不能用复数are necessary.

     因此，综合以上分析，可以得出这样的结论：当比较对象为单数时，要用单数than is necessary. 一般不能用复数。比较对象为复数时，than is/are 都可以。

     请看几个这样的例句：

1. Social and economic factors should be considered in deciding whether to adopt controls which are stricter than is necessary to maintain water quality. （比较对象controls（复数），than 后却用了单数，当然如果使用复数也可以）

2. The pillars left in such layouts are obviously much wider than is necessary for stability. (比较对象复数pillars， than 后用了单数，当然复数也对）

3. More items on some sub- tests are administered than is necessary.（比较对象Items (复数），than后为单数，当然复数也对）

为了简练，有很多人直接使用more than necessary (省略be动词，这也是对的）。

M. Swan的《牛津英语用法指南》认为，than 后省略代词时可以看作关系代词。该书例句：She spent more money than was sensible. (not .. . than it was sensible.)

Swan认为后面必须省略it. 按照我的理解，作为权宜之计，简单理解为相同比较对象money要省略，避免重复。实际上应该是相当于than it was sensible that she should spent the money. 如果比较对象不同，就不能省略了。如：She spent more money than her sister.