I doubt Blizzard considers multiboxing for encounter design at all. It's a stretch to say that they would build them specifically to inconvenience a small, ultimately somewhat insignificant portion of the playerbase instead of just designing what they think would work for the majorit. It's simply that encounters are more and more movement heavy and there is less focus on micromanagement of your spells and abilities to maintain good output.

I tried the first part with another 10-boxer, but for some reason it felt like we were not doing any DPS. My 10 BDKs were bringing it down considerably faster on normal mode...