Having it be instant cast can be a huge benefit, since you don't have to even be facing a target to cast affliction spells. Anytime you run away from something being able to dot and tap/Dark Pact, has always seemed more then worth the investment.
"I have nothing personally invested in my own opinions. I'm just, like, inviting you to join me on the bandwagon of my own uncertainty." -Taylor Mali
I was just thinking about this today, as I currently have 5/5 in the talent. As mentioned above, it's great when you're on the go - when farming the eels NW of Shattrath, I typically swim around hitting Corruption, Curse of Agony, and Siphon Life then tabbing to a new creature. I never have to stop until I run out of mobs or until I want to loot. Not having to face mobs when you're casting instant spells is a very powerful ability, and contributes a lot to my enjoyment of the Warlock class, funny enough.
In a more serious vein, I think you could get by with fewer points in it, but it's difficult to get to 41 points in the Affliction tree without doing so. The only way I was able to get there was by putting 5 points in Suppression, which was also something I was trying to avoid (given I was trying to avoid putting points in Shadow Embrace *and* Malediction too).
So I guess the bottom line is: if you're not Affliction heavy, you can certainly get away with putting 1 to 3 points in Improved Corruption. For an Affliction Warlock, however, it's much more difficult. Personally, I love having it as instant cast and can't imagine changing it.
If in the course of a lengthy fight, when you are trying to keep all your dots up as close to 100% of the time as possible you get into this scenario:
(pretend this an accurate picture of your dot timers )
1.8 sec Corruption on Blah
1.5 sec immolation on Blah
This might not happen as part of your "perfect" rotation, this might not be ideal, but this will happen. What do you do? In theory, you want to start to cast a new dot so that it goes off just as the previous cast of it falls off, but in this scenario you have 2 dots that are going to fall off very close to each other and its impossible to start to cast them in such a way to have no down time, unless, of course, your corruption in an instant cast.
If you're moving, you're not casting anything else anyways. So refresh corruption (even if it isn't down) and lifetap the mana back (works as long as you're moving for at least 3 seconds) -- little reason not to, unless you're very health-constrained, and you can save time later you would've otherwise spent refreshing corruption.
See also: siphon life, curses, shadowburn (if you're good at jumping and turning -- if not, it's easy enough to learn.)
Using it in that way does gain you damage over time from the points, even if it doesn't change the dps of the spell in a perfect, standing-still, theory-crafting world.
Gruul while flying/shatter. Aran while running out of blizzard. Prince while repositioning. The channelers on Mag if you can afford the global cooldown while you're moving towards an infernal that's about to break CC. etc.
It also comes down to, where else will you put those early affliction points? Too many points in suppression is of dubious value since you need +hit for soulshatter and shadowbolts also. I'm not a big soul siphon fan (maybe if we had 40-man raids with lots of warlocks and I was using drain life a lot on raids, but.) You do need to find a home for 10 points in the first two tiers before you can move on.
(edit) I should add, I always have affliction points in my raiding builds, because I find range and -threat talents to be too important to how I'm happy playing. So the "where do I spend 10 points" comes up a lot. I'm 42/1/18 now. I was 21/0/40 before.
(edit2) There is something to be said, however, for only 2-3 points in for the ease of being able to cast one dot and then just hit the next. I pretty frequently mistime the global cooldown but I'm great with timing my spells. (thinking about this, it's probably because I don't have a cast bar for the global cooldown. I wonder if someone's written a mod that makes a cast bar. Hmm. Off to troll wowace.com.)
As redundant as this may be (I've known it for a while, but I *know* I read this on a different thread here at EJ in the past two weeks) taking Improved Corruption increases the damage done in a short term fight (ie, trash) of Corruption because the damage starts ticking three seconds after the mob is afflicted. By taking the talent points, your damage done per trash pull will increase (albeit, only 1 tick) and over time, this damage will add up as a major contribution to raid DPS. Still, I feel the mobility factor for both farming, PvP, and PvE fights where you need to be constantly moving outweighs the damage. Nothing more than a perk.
I wouldn't want to play without a good instant damage spell without drawbacks. Without talents, we only have two instants, a curse and deathcoil. Making more spells instant is good, especially at a low cost of talent points.
Another use of instant corruption is the boss of Heroic BF, he does a "get over here" move and then you must run or take a lot of damage.
Ofc the PvP usage makes the spell invaluable.
Sorren's Timers (on wowace) shows a cast bar from the GCD.
It gives you an entire global cooldown that you can move without losing DPS. It also opens up Improved Life Tap to the non-affliction locks (Who really don't need many points in suppression compared to deep afflic locks).
Don't forget that on fights with constant AE damage, without 5/5 improved corruption, you're going to take pushback while you cast it, and lose more than a few seconds over the course of the fight.