It is not a requirement to keep your boarding passes until the end of the flight, in fact most people throw them away during the flight. They should hold on to it tho if their baggage receipt is attached in case they end up with no baggage at their final destination, however even this is not completely necessary.
The hostie should have had a pax list and along with your ID/passport be able to confirm who you are and where you should be sat. This is nothing to panic about, maybe she was new or the captain was putting her under stress to depart quickly and this was a hinderance.
If you had no ID, there were more people on the a/c than there should be or your name was not on the pax list, that is when one would get concerned, even then it's probably a check-in/boarding error (quite common) and not really worth fretting about. As long as it's sorted prior to departure who cares?
Could just be a typical f/a over-reaction, a common trait within the profession. (Yes I am one so I am qualified to say that before you start moaning he he!!)
ATB, T
