What about if the kid wants to get a Lexus for his first car. Both parents think they are safe reliable, la la la and are in for half. That is a choice that does not necessarily affect the home if it can be afforded BUT it does potentially affect the marriage if the "step" thinks its excessive and that the money spent on said car would be better invested elsewhere. It's not as black and white as you people are making it.
The scenario you gave is indeed black and white:
The step butts out.
Just because the step's opinion is that the money could be better spent doesn't make it so. To insist and make a big deal out of it is just some juvenile power play, especially if the money is there.
Ideally all married parties have their own separate "mad" money that they can spend as they please. To not do so is dumb.