I've seen this limitation noted by a few ISPs, but being honest, I don't think they can apply it. Eg, I used to use a dialup connection with one of the ISPs who (certainly used to) quote this clause, but my CLI is disabled (my job means I don't want clients having my home number, and dialling 141 before the call was error-prone). Absolutely no probs. I think it may just be a case of "1 account, 1 SIMULTANEOUS connection", and a fairly cack-handed way of trying to stop several people using the same account.
The only exception, now I come to think of it, is with mail accounts, which typically will not accept sender end mail from a different ISP. Example - if I use Outlook with my BT Openworld account, but dial in using freeserve, BTO get arsey and refuse the post... the reverse is also true.