Arelith

Is there any practical point in taking IDK on a large race? Other than the occasional Dragon humbling?

It makes the large character count as a huge size category during knockdown attempts.
As I understand it, this would translate into (-4+8)= +4 AB during their IKD attempts vs medium-sized targets while giving them -8 to their discipline checks.

As XXX pointed out, it's generally a significant AB boost you receive. It's in the same vein of people taking Improved Disarm when they primarily use a 2-handed large weapon.

Knockdown/Improved Knockdown will always be made at a -4 AB, it is the discipline opposing checks that are made with size category in mind.

Only disarm and improved disarm are made with AB differences along with discipline differences based on opposing weapon sizes.

Example: A firbolg with 52 AB and IKD attempts to knock down a halfling, large size vs small size made it 50 AB + d20, let's assume firbolg rolled a 15 which results in 65 AB.
The knockdown attempt is then made at 61 AB, the discipline check for the halfling is 73 due to size difference of huge (5) vs small (2) which is 3 * 4 = 12.

Thanks all!