All-X strategies do mean that your economy is efficient and less wasteful, but these strategies mean that your spending is mainly on one thing, and if you have to then change your focus to something else well then you're not really All-Xing anymore.
An economy which gives you a bit of everything you need may be wasteful, but it allows you to progress militarily and technologically at a reasonable rate. And, you can set the rate of either to suit your needs without having to build a whole new lot of infrastructure first.
More than that, you can control planetary economy by demolishing infrastructure you don't need at the moment, or by not building ships there for a while. The first of those measures means you save maintenance and don't have so much spending, the second means that spending gets refunded because it isn't used. Both are measures that can save you money to use elsewhere, and make your sliders more effective. But to some, the concept of not building, or tearing down buildings, is a little difficult to grasp.
Think on it this way, if you're going to make a breakthrough which changes a lot of ship designs, you're going to need some cash to upgrade your existing hulls. So telling the shipyards to stop building can give you bcs that you need. When you actually have a new design, you immediately have idle starports ready to build that design, and cash to upgrade the obsolete ships.