I get the objection to spending money on things that don't provide tangible benefit...but I don't get it at the same time.
First, there are (simple) rules in the DMG for sinking cash into real estate, strongholds, followers, and businesses. They are rather PC focused, because you have to sit around doing nothing but running them or take a loss, which isn't reasonable in all situations. House rules required for more verisimilitude. But simple rules are provided.
Now, back to the worlds of D&D. I'm of the opinion that attempts to separate mechanical from non-mechanical benefits are futile, since you can get mechanical benefits from anything. If you hire 4 bodyguards, you have 4 more stat-blocks fighting for you in a battle: mechanical. If you spend you money making friends and influencing people, those people might be able to help you out with things that very much fall within D&D rules, like opening locks or using knowledge skills: mechanical.
But it's just hard for me to even separate these things. I have money. Bribe the war chief to go fight one of your enemies instead of the people you are trying to protect. That's a whole lot of hit points saved: mechanical.
Money is highly fungible power. You can turn it into just about anything. Even if it is hard to find a magic item shop, you can probably hire sages, historians and diviners to find out where the magic item you want might be found. Now you have an adventure with a goal you want to accomplish.
The only meaning I can actually ascribe to the objection is that it isn't something you, as the player, get to roll dice on. It isn't a number or special power on your character sheet. I suppose a DM could write things up in that way, and maybe they should if that's what's needed to get people to see how enjoyable that part of the game is. But really, writing it up like that just takes away from the creative power of the player, because it implies (hopefully doesn't mandate, but the implication is there) that you have to have it written down to do it.