Christ on a bike, why is everyone only seeing the extremes of these points of view[1]?
I'm sure the people favouring a consultative approach to major purchases have a threshold below which they simply buy (a spectrum running from replacement brakepads, perhaps to a point below that n+1).
Equally, I'm sure the opposite side have some sort of threshold at which purchases are significant enough a dent in the household income to merit discussion - that might be high enough to mean a new bike can come out of petty cash, in which case, GOOD FOR YOU, well done on doing so well.
Jeez, some folk might just like talking through an idea to help cement their own "need" or otherwise for the item (a bit like the coin toss, in which the point isn't so much to let the coin decide, as to know, the minute you call heads or tails, what you really want).
[1] Is it the internet? I bet it's the internet.