Fewer Set Items Required for Bonuses = More Gear Variety?

Sets have 6 parts but you get bonuses for using 2, 3, and 4 parts instead of the full 6. This means that you have a much larger degree of control over which items you use. Now if you find a sweet pair of Frostburns or Andariel’s Visage you can easily use it if you wish, even better, you can use both!
Tyvalir: Thanks for this bit of feedback, Silentstar.

One of our ongoing goals is to ensure that there are a variety of items which are valuable for each gear slot, and we have been adding new Legendary and Set item affixes to the game over time to this effect. Aside from making more items competitive for each gear slot, this process will also naturally result in players having to make more difficult choices about which items to equip as time goes on.

Nevalistis explained this philosophy a bit further in a previous post:

We decided against adding shoulders as one of the new pieces while expanding the older sets to 7 pieces. One of the reasons is because many builds do not currently have a game-changing legendary in their shoulder slot. Of course, this is exactly why people want the new set to be a shoulder item – it would make for a very easy choice. However, we want there to be some contention between your gear choices, which is why we’re looking more at slots like gloves, rings, and bracers. We are deliberately trying to create more difficult item choices while leaving some flexibility during the gear-up process.