Remember Gurthalak? Remember how it single-handedly propped up arms warriors at the end of Cataclysm? The proc on the weapon was really excellent, almost any melee who could use a 2h weapon wanted one. Admittedly, Gurth didn't invent proc weapons. Icecrown Citadel had some excellent weapon procs, Last Word, for instance, had an excellent proc primarily because it was almost always up - a proc that thematically owed a lot to the first real dedicated tanking epic warriors in Classic WoW would have wanted to get, Quel'Serrar. Quel's defense proc (back when defense was a thing) made it very desirable as a tanking weapon, second only to Thunderfury - and the legendary was so amazing because of its proc. It was essentially a super-Thunder Clap, great for aggro and for debuffing attack speed on targets, reducing damage taken by the tank using it.
Proc weapons have a long and stories history in World of Warcraft. One of the first weapons I crafted at level 60 was an Arcanite Champion, a weapon based entirely around its strength and heal proc. It replaced the Blackhand Doomsaw, another proc based weapon. As time has progressed, we've moved away from these weapons - stat based weapons have dominated, and for the most part that's for the best, as it's easier to balance stats. If one looks at the evolution of proc weapons, even in Burning Crusade those weapons that had procs still tended to have stats on them.
One of my favorite things about Dragon Soul was the proc weapons on Deathwing - they seemed special, designed less around playing statistical Tetris through reforging and more around visual flair as well as interesting damage output.