Back in the day (well, back even a few weeks ago), Immolate played a significant role in Destruction's damage. It used to be mandatory to cast Conflag on a target, and it used to increase the damage of our nukes (this was primarily a dmg throttling mechanic introduced after S7 arena and Destro actually being deadly, and prob a bit OP). The 1.5 (originally 2sec before talents) cast time was basically the price you paid to get a Conflag off. I can't say that we always agreed it was necessary (and probably the main source of Destro's pvp woes thanks to dispels), but at least you could see an arguement for it.
Today, Immolate does add a snare to the target when you Conflag, but this is basically negated 100% by the glyph (which any pvp Destro lock, and many pve Destro locks will grab). What we're left with is a so/so DoT that's just kind of there. It's damage is comparable to Living Bomb and Flame Shock (our 2 closest DoT peers here - Moonfire probably would fit too, but it's the source of some special spec-specific love), it's secondary effect is equal to or worse than LB(AoE dmg)/FS(insta LB procs), yet those DoTs are all instant and ours is not.
Why, in Destro's current implementation, does Immolate *still* have a cast time?
I think it's clearly a relic of the previous design model, and based on every similar spell in the game - it should be changed to instant cast. Note: this is neither a dps boost or a significant pvp buff. It's mainly a QoL change minimally buffing our mobile dmg dealing (and really only in lvling/bg/5man environments). I'm just sick of seeing a cast bar attached to Immolate, when every other primary Caster DoT is instant.
It's time to get with the times.