This was also an issue with Marvel Legends before. Many people complained why ToyBiz was wasting slots putting Wolverine, Ironman, Spiderman, Captain America, Hulk, etc. when they should be making new characters. I read that putting one of the top 10 in every series is important. Not everyone knows secondary characters but kids know Wolverine, Spiderman, Captain America (lets consider kids also as a target market, not only collectors). For ML, they only need to put one top 10 guy in every series. But it's a different story for DST.
I think the assortments were planned carefully. Take the Captain America and Arnim Zola on different perspectives for example;
-Collectors don't need another Cap, but they wanted a Zola. They buy the pack, get Zola and throw Cap to the fodder bin, or sell him.
-Kids love Cap but they don't know Arnim Zola. They ask mommy to buy the pack to get Cap, and the bonus Zola is an instant villain, or just throw him to the toybox, or give it to a friend. lol.
With this type of strategy, DST can cater to both collectors and non-collectors at the same time. It may seem as overkill putting Cap, Iron-Man, but it's necessary for the packs to sell... and the line to keep going.