I prefer to think of it as dancing on the head of a needle: you might fall off in any of the 4 cardinal directions (aka into worship of one of the 4 chaos gods at any time) and there's a very real danger of slipping and impaling yourself on this precarious perch, but in the mean while...
Well, you're living for all you're worth.
There are more traitor legions that are generally Undivided than there are devoted to chaos gods (though worshippers of Chaos, much less CSM in general, are by no means solely made up of the 9 original traitor legions)--4 legions devoted to individual gods vs 5 that worship "general... chaosiness."
What sphinx of plascrete and adamantium bashed open their skulls and ate up their brains and imagination? Imperator! Imperator!