At least in socal Pacific Bell territory, ISDN didn't have an unlimited tariff, so you'd be paying per minute when you were online, and a lot of ISPs charged more for ISDN as well.
My house only supported 24K modem speed due to loop length but I discovered they (Qwest) could provide basic rate ISDN. There was a tarrif that allowed unlimited long distance connections and so I dialed into UUNet in Virginia, permanently. Latency was excellent.
