I can only agree.
Being a fan of DoT management I wanted to experiment a cross affliction/shadow build only to realize that there is little to no viable options for spreading DoTs on trash packs, which is a big part of the content.
Beside the Pestilence RE which is not very practical just to spead one DoT, there's also a huge issue in the affliction talent tree.
Contagion is supposed to spread Corruption on its last tick with the help of Drain Soul, however Everlasting Affliction (which can't be overlook) makes Drain Soul reset the timer of Corruption. These two talents are literally working against each other.
And that's once again just to spread one DoT ~
A personal suggestion would be a RE or talent that spreads warlock DoTs from Seed of Corruption targets, and later do the same for Shadow priest DoTs when Mind Sear will be available. It could also be a Mystic Enchant working like the Tainted Swipe RE
Obviously if this mechanic gets out of control (possibly in PvP), spreaded DoTs could just be weaker versions of the ones present on patient zero.