I am from far enough back in time (started with 1st edition and then went to 2nd - and had almost all of the codexes, even though I only played a single faction/army) I would buy codexes (army books) for all the armies, because I liked the art and the lore.

The 2nd edition box set was about ~£35 in 1993, adjusted for inflation that would be ~£73 now - which then when converted into CAD is well...alot more than what I just paid for 10th edition (about $80 CAD+tax). So - it's a good deal - and I am sure that there is overlap amongst friends during edition changeover.

5-year cycle would be a happy medium, but "that's just like my opinion man"...