That or who

You can use either 'that' or 'who'. You must use one or the other.

As the relative pronoun, either who or that is possible but either one is not optional, ie. it cannot be dropped.

Beat me by a minute, centrox.

You can go with either I know many people that think he should resign or I know many people who think he should resign.

The use of the word that later in the sentence is not necessary.

Why?

The entire message is conveyed without the word that being added. It is stated, "many people think" the person should resign.

Since it IS optional, "The entire message is conveyed without the word that being added" seemingly makes no nevermind, wouldn't you say, Sturgis?

Using the empty complementiser 'that' is optional. Opting to not use it is slightly more idiomatic than using it.

Grammatical choices are not "empty". If they are grammatical, and this one is, how can it be considered empty?

That doesn't sound very academic. That sounds very emptily prescriptive, which, as we all know, means empty.

Many grammatical options are retained for the more formal. Are these then to be considered 'empty'?

What is wrong with using science as regards these issues?

Agreed as to "more idiomatic than using it" though I'm not sure on the "slightly" portion.