Why does measuring for Asymmetric Binary Attributes exclude t (Values which are 0 for both)?
I know it is the old post, but I also search for the answer to this question. and here what I got:

first of all, binary asymmetric attributes only care the 1 values. for example in measuring the distance of the patient based on their symptom, 1 means the symptom exists and 0 mean it does not exist. Or another example in measuring the distance between 2 documents based on the occurrence of words/term (most of these cases not using binary, but still asymmetric), 0 means the term does not exist in the document.

in those 2 cases, the number of symptom or term can be very huge and may cause a lot of 0 values appear in 1 records. This is because one disease may only have a small number of symptoms or one document may only contain a small number of words/term. If all of these 0 values are used to calculate the distance between two records, the similarity can be huge. example

record 1 :  1 0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

record 2 :  0 0 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

assymetric:
d(1,2) = 3/4  --> similarity(1,2) = 1-3/4 = 1/4

IF we consider it as symmertic:
d(1,2) = 3/20 --> similarity(1,2) = 1-3/20 = 17/20 (large similarity)

that is why the attributes with the same 0 values between two records are not included in the distance calculation.