I think he felt he had to do it that way because they all had to come together at a certain time, and I think after reading Perrin's story in ToM we would have wondered "Why didn't Tam mention this to Rand? Did it just slip his mind?"


the only problem with that is he was so nonchalant about it that it really didn't need to be said at all. he could have said "it's a really long story" and then rand could have started to be an a-hole shortly after and we never had to find out about morgase. tam had no reason to think rand would care about morgase, so it's not like it was dire that he tell him.