Is a consistent design necessary?

The article The importance of having a design system by Reindeeraintreal (Reddit) shows the following UI elements from the gaming platform Steam.

The various UI elements of Steam

That looks pretty bad, right? There is no consistency at all.

However, the first comment is pretty astute: “Steam, being as successful as it is, makes me interpret this post as saying that having a design system is not important.”

Design’s not the only thing, that’s for sure. Market dominance definitely trumps design. But you could do your users a favor and lower the impedance incurred by using your product. In these days of security holes and phishing, a consistent design helps your users make smarter decisions, by default. If users can’t tell what your standard design is, then they can’t be alert to times when the standard design is not followed.

It’s kind of like when the U.S. had 3 (or 4?) different kinds of 20-dollar bills in circulation. That opened the door to counterfeiters not even having to try real hard because you could just say “oh, that’s the new $20 … don’t worry about it. I just got it from an ATM.”