Plague Leech was not mandatory throughout MoP. It wasn't worth using when it only restored 1 rune. When it was buffed (in 5.3? I think?) it was worth using but was still a fairly small performance increase. I personally hate Plague Leech and refuse to use it. I'm not being disingenuous at all-- I didn't use it in MoP, and I won't use it in WoD.
That's just me personally. If you like Plague Leech, downtime will be much lower for you.