Context context context. Might have helped you guys.

I role PVE, mostly instances. 5 man mixed class group (Paladin, Druid, Warlock, Priest, Hunter)

I strive for single button DPS at almost any cost, hence the huge macros.

I have challenges that I want to overcome on my hunter, priest and warlock.

Each has spamable, non-cooldown spells (Shadow Bolt, Mind Flay, Steady Shot)

Each has cooldown spells that need to be inserted between the spamable ones when off cooldown (Chaos Bolt, Mind Blast, SW, Devouring Plague, Arcane Shot, Multi-shot)

Each has a very few spells that need to be applied maybe just once (Hunter's Mark, SW:P, Curse of Elements)

Each has a spell that needs to be applied periodically (Corruption, Vampiric Touch, Scorpid Sting)


Though crazy complicated, I strive to fit all of these abilities into a single one button, do it all, wonder-macro. Here's my thoughts based on your input. I like castrandom because it deals with abilities with cooldown, but I dislike mixing castrandom with castsequence. Here is my thought on this for the priest.

Code:
#show Mind Flay 
/stopmacro [channeling] 
/cast [nostance] shadowform 
/target [target=target, harm, nodead][target=party4target, harm, nodead][target=party1target, harm, nodead]
[target=party2target, harm, nodead] 
/castrandom [combat] Devouring Plague, Shadow Word: Death 
/castsequence reset=target/combat Vampiric Embrace, Shadow Word: pain, Vampiric Touch, Mind Blast, Mind Flay, Mind Flay, Mind Blast, Mind Flay, Mind Flay, Vampiric Touch, Mind Blast, Mind Flay, Mind Flay, Mind Blast, Mind Flay, Mind Flay, Vampiric Touch, Mind Blast, Mind Flay, Mind Flay, Mind Blast, Mind Flay, Mind Flay, Vampiric Touch, Mind Blast, Mind Flay, Mind Flay, Mind Blast, Mind Flay, Mind Flay, Vampiric Touch, Mind Blast, Mind Flay, Mind Flay, Mind Blast, Mind Flay, Mind Flay
The reason make the sequence so long is so that Shadow Word Pain doesn't get re-applied and so that it's up on my targets.

I'm late for class and will have to post the warlock thoughts later.