Every time one of ur Disease ticks it has a chance to proc Wandering Plague (WP).
If they proc in the same time, which they do when u use Glyph of Disease, there is a chance WP will proc every 3sec.
If u cast IT > PS or PS > IT, WP will proc every time Frost Fever ticks and every time Blood Plague ticks.
(aslong as they tick one after another, aka not in the same time).
So basicly there is a chance WP will proc 2 times every 3sec.
(this is just an atempt to make u understand how this works)
On top of that, IT + PS + BS deal more DMG than SS + Pestilence if SS doesnt crit.
U should use: Glyph of Icy Touch, Glyph of The Ghoul, Glyph of Dark Death.
The only spec and rotation in which u should use Glyph of Disease is if u go Unholy Full armor penetration Blood Subspec build.
And from what i know that spec sux...