(...) If there's no big common cost in the near future (think new car, house, school fees ...) and you are in a stable position income-wise, and you have the personal budget for a 1000,00$ wheelset or purse, why would you need a discussion about that, however stupid you think the other 's spending is? That is the freedom you should be able to give eachother.
In my case my wife doesn't have an expensive hobby and doesn't like jewelry nor other expensive stuff.
We usually discuss before I buy expensive cycling things, just because right now money is there, but who knows if in some years for example we will have kids, temporarily loose job, etc...
In 10 years I won't remember those $400 brakes, but it would be a shame not to have saved money for ex. to build a house just because I bought new carbon wheels every year.
You have to think a bit long term and even if I know nothing about economy and finance, common sense tells me that financially-wise most money spent in cycling stuff is wasted (i.e. not an investment).
Discussing with my wife helps me keeping things in perspective: she may help me realize that some uber-expensive toys (that for me and my cycling-obsessed friends are normally accepted) are really stupid in fact.
I would also say that we (me included) are victims of a huge marketing operation: years ago people realized that cycling is a very good way to make money and understood that people are ready to pay big money for useless things if you tell them hard enough that they are needed to enjoy the sport.
Just read one of the Italian Granfondo magazines to realize that.
...but this is an entirely different discussion