For people with multiple Discover cards, it would be a big headache to need multiple IDs and passwords.
I just set up my Brooks Bros online access with Citi. The website is a little more elaborate than Synchrony's, but it also seems a bit buggy. I had to enter my checking account info twice, and some features don't work.
Of course, if I can get more Amex offers in the future, it won't bother me that the store card is imperfect.