I'm a little confused by your comment - Airlines do re-use flight numbers - BA988 is always the BA flight from LHR to BER at around 19:30 (i.e. there will be a BA988 today, there was one yesterday, and there will be another tomorrow).
But to be honest, I'd also say it's not that useful for me personally. It's a lot easier to remember your destination and fight-time - (i.e. I'm on the 19:30 to Berlin) than a flight-number, and only once do I think there was ever the potential for confusion).
[You experience may vary, especially if you are are used to flying a busy flight corridor.]