Living within your means at any level is the most important thing, but whether or not you have the ability to buy useless things or invest at either level isn't the larger capitalist point. The difference lies in how much of a worthwhile debt risk you're seen as, to be "trusted" with credit either buy useless things or invest. Apply for the same loan under different names, list the different incomes and see the responses you get.
How you leverage debt, how you manage risk, how you live within your means matters. All that other garbage is just distraction.
And it's a very good idea to not let yourself be trapped in that because-they-said-I-can game either, and have other options.