I dont get it

[Vidara]

Because the more time you spend outside DS channeling, the more ticks you miss...

Its so simple that Immolate isnt worth casting cause it doesnt offer enough bang for the buck compared to the alternatives below 25%.

[daveyman]

But it does though. That's the thing.

DPCT of Drain Soul under 25 percent is: 5887.7 (geared and buffed)
DPCT of Immolate is: 7170.85 (geared and buffed)

Cast it. It is a DPS increase. How many times can it be said. The fact that it doesn't help your DS is irrelevant.

[Nam]

That's assuming one point in molten core and Glyph of Immolate as well (which most affliction warlocks will have). The DPCT for DS assume 15 affliction debuffs on the boss, so it's even lower when you are the only affliction warlock.
Nevertheless, the difference is small. Casting Immolate is not a mistake, not casting Immolate is, however, not a critical thing. You'll loose some dps in the last quarter of the fight, but have a little less trouble with drain soul. I personally reapply Immolate, but I've seen very good wws stats where people didn't, and others where they did.