My CCG experience says you cannot use an "Action:" abilties while exhausted...but I cannot find ANY reference in either book that says you cannot!

It makes a huge difference. Consider this combat example, where Player A has Initiative:

Player A has a Tzeentch Flamer. He exhausts it to deal 2 damage to a target unit.

Before Player B has a chance to strike, an Action Window opens. Player A gets the first Action, and uses the Action ability on the Flamer to sacrifice it, doing 2 more damage to a target unit.

Other than Shields, there seems to be no way to stop this simple combo, simply because nowhere does it state that you must be Ready to use an Action ability.

Now, I know that SOME cards include Exhausting as a cost, but not all, and its this that I'm talking about.

Can you tell me if I'm wrong?

Kealios

ps Reactions seem to be the same - unless you have to Exhaust, you can use a Reaction at any time, Ready or Exhausted...

Correct. If you meet the play restrictions and can pay the cost, you can trigger the Action (or Reaction, or Interrupt, as the case may be). There is no blanket rule saying you can only trigger the Actions on ready cards, or "turning off" card text when it is exhausted.

So yeah, unless the card requires you to exhaust as part of paying costs (or has some other text requiring the unit to be ready), you can use its abilities when exhausted. And yes, this makes the Flamers incredibly good. It also means the bonuses on Void Pirates and Rogue Traders in your HQ count if you win the command struggle at the planet they head to with your warlord.

What you guys say makes sense to me.

In Magic the gathering terms Flamers would have:

T: deal 2 damage to target creature.

and

Sacrifice Flamers of Tzeentch to deal 2 damage to target creature.