If you’re building any product with a human interface – be it software, a web-application, mobile-app, even physical hardware: you need a full-time designer.
Sum of the parts
Take the analogy of evolution: you’d imagine that to win the game of natural-selection an animal would want all advantageous features to be maximised: i.e. smartest, strongest, fastest, most-reproductive – but you can’t simply increase one attribute without affecting another. The outcome of the system is about the combination of all its properties: i.e. a system is a whole, representing the sum of the internal relationship of its constituent parts. Change one part, and there are waves of impact on the rest of the system.
And so it is with your product. Introduce a new feature; remove a navigation element; change the colour, size or prominence of X, Y or Z – all these seemingly insignificant decisions that might be led from engineering / business-oriented concerns will impact the total usability and intuitive coherence of the system as a whole.
The Do-Once Done-With Fallacy
Design is not a “do-once, done-with” proposition – it needs to react and reflect each change in the system; it needs to iterate and morph alongside the codebase and feature-set itself.
Many non-designers have the general idea: “We’ll spend 9 months building it; then we’ll hire a crack contract designer for a week, to Make-It-Look-Pretty”. 9 months in however, the vast majority of the essential, vital design decisions have been made. These decisions might not seem like design, they might seem more like plain old logic and planning: “Navigation goes here”, “clicking this button opens up this widget” etc. but the discipline of design is not about Making-It-Look-Pretty, but about allowing a human-being to interface with your product in a frictionless manner. And once these design decisions are baked deep into the architecture of the code; they are a lot more costly to undo.
Often this leads directly to: “Aha, well in that case, we’ll hire a crack designer to plan out the UI / UX of our product at the beginning of the build” — but this doesn’t work either.
Anyone, who has built anything, ever: discovers they didn’t get it right the first time; your product will need to be revised, reworked and modified, a number of times, before you get it right (and even then, there will always be room for improvement and new directions). But every modification has the potential to negatively impact the intuitive coherence of the product as a whole.
Do It Right
So: if you think you can do without design, or can just deal with it in a sandbox kind-of way: you’re doing it wrong. If you’re building a product, or thinking of building one, put a full-time designer as high on your list as a full time engineer.