I don't believe the "guaranteed" idea should be generalized, in that most people do expect a legendary item to drop within a given amount of time for the drop rate to be acceptable (simply another way to look at guaranteed legendaries). Blizzard doubled the rate for legendaries to drop, creating a higher potential guarantee for legendaries dropping.
As I've sort of said, most people expect a legendary to drop within a given amount of gameplay time. Adding in special monsters that have a 20% chance of guaranteed legendary drop doesn't actually produce a guaranteed drop--it creates just another opportunity for players to feel excited about the hunt.
If a player successfully runs into and kills three special monsters and three legendaries drop, it would be equivalent to a player finding three legendaries without killing the special monsters, except in the first scenario the player actually did something to warrant a legendary dropping instead of randomly killing crap hoping for RNG's favor.
Putting in monsters to hunt while grinding through crap and champ mobs would increase player satisfaction for finding legendaries. Right now, players hope to randomly hit the jackpot and have a legendary drop. At least in Diablo 2 players could feel accomplished for killing bosses when uniques dropped. Logically, bosses should have better items than their minions, and special monsters would indeed carry better items, too--all the more exciting to hunt them down.