Defender is an oddball among current, evergreen keywords: It's a keyworded drawback, which generally belong to the distant past and a less progressive era of Magic design (see the general unfun of echo and cumulative upkeep). To be fair, it is sort of a relic itself that was converted into a keyword only when Walls (and other creature types, like Legend) lost their inherent rules meanings, but it's oddly well-liked for something that makes creatures worse. I have a soft spot for it myself; maybe it's because of the self-imposed challenge of winning with creatures that can't attack (i.e., can't do the thing that most usually wins games) appeals to me.
Defenders also slow down the game merely by existing in an environment, which is beneficial for building an intentionally slow, "battlecruiser Magic" or otherwise, format. That's why we saw a defender subtheme in Rise of the Eldrazi - and in Magic 2015, and Mark Rosewater recently hinted on his Tumblr blog that there may be one in Khans of Tarkir.
When web-brand asked, "How much design space is there in 'defenders matter'?" Rosewater confirmed that there was still some. Many of the comments seemed to suspect that the "next block planted theme" in Magic 2015 was indeed Walls and other defenders, as enchantments were in Magic 2014 for Theros block, and so on. Tarkir is definitely a bottom-up, mechanics-first block, and defender certainly does not a full block theme make, but it fits as one small part of a larger, overarching "combat matters" theme that most people suspect Tarkir to have. Defenders as a "combat style" is a good way to differentiate a real "combat matters" block from the others where, well, combat always matters, and who could pass up a chance to have a Great Wall of Tarkir in a Central Asian-esque world?