Call me crazy, but I *think* the idea is to build a list BEFORE you know what you'll be fighting against...
Seems sort of unfair to say, "Well, I'm fighting a ton of Orks... I'd better take 27 flamers." Or something like, "3 Land Raiders? I guess I'll bring 9 Broadsides!"
Anyhow, the tailoring of the list based on what you'll be fighting seems to be common in the newer players. It's probably in every bodies best interest if it stops.