Do we really need to use 했던?

My grammar book (외국인을 위한 한국어 문법2) defines 던 and 했던 the same way:

명사를 수식하게 하고 과거 상황을 회상하거나 그 상황이 완료되지 않고 중단되었음을 나타내는 표현.

It modifies nouns and is used in expressions to show reflection on the past or to show a situation that was uncompleted or discontinued in the past.

Since 던 and 했던 mean the same thing, there is no reason to use 했던. All the example sentences using 했던 in my grammar book can be written with 던. (The English is my translation.)

1. 어릴 때 한 동네에서 살았던 민수를 어제 만났다.
   어릴 때 한 동네에서 살던 민수를 어제 만났다.
   Yesterday, I met Min-su, who had lived in the same village as me when we were children.
   .
2. 작년에 읽었던 책은 어제 다시 읽었는데 여전히 재미있더라.
   작년에 읽던 책은 어제 다시 읽었는데 여전히 재미있더라.
   Yesterday, I read a book I had started reading last year and found it still interesting.
   .
3. 내가 전에 근무했던 회사가 아주 많이 발전했더라.
   내가 전에 근무하던 회사가 아주 많이 발전했더라.
   The company I used to work at has grown a great deal.
   .
4. 고등하교 때는 키가 작었던 영수가 지금은 몰라 볼 정도로 키가 컸더라.
   고등하교 때는 키가 작던 영수가 지금은 몰라 볼 정도로 키가 컸더라.
   Yeong-su was short in high school, but he has now grown so much that I didn't recognize him.
   .
5. 어렸을 때 예뻤던 순이가 지금은 아줌마가 다 되었다.
   어렸을 때 예쁘던 순이가 지금은 아줌마가 다 되었다.
   Sun-i, who was pretty when she was young, has now become an average-looking housewife.
   .
6. 10년 전 초등학생이었던 순이가 벌써 결혼을 한대.
   10년 전 초등학생이던 순이가 벌써 결혼을 한대.
   Sun-i, who was an elementary school student ten years ago, says she is already getting married.

Notice that 던 replaced 었던 in the above sentences without any change in meaning; therefore, why bother learning the 었/았/였던 pattern?

As mentioned above, 던 can be used to show not only reflection on the past, but also to show that an action was uncompleted or interrupted in the past. In Example 2, 읽던 책 means the person started reading the book in the past, but did not finish it. It would be translated as "a book I had started reading (last year)." If he had wanted to say he had already completed reading the book (last year), he would have said 읽은 책, which translates as "a book I had read (last year)."

See the following examples:

1. 어제 마시던 우유가 어디에 갔지?
   Where is the milk I was drinking yesterday?
2. 어제 마신 우유가 무엇이었지?
   What was the milk I drank yesterday?

In Example 1, the person did not finish drinking all the milk yesterday and wanted to drink some more today. In Example 2, the person had drank all the milk yesterday and is curious what brand it was.

Another Reference: "Is 던 better than 한?"