Why designers should be informed is a simple line of reasoning. It’s based on the premise that any design solution is the culmination of a series of decisions — “Should this button be blue or red?”, “Should I use Helvetica or Baskerville?”
“…any design solution is the culmination of a series of decisions…”
It follows that the quality of the final design is based on how well each decision was informed: “I’ll use blue because it contrasts nicely with the background without being garish”, and “I’ll use Helvetica because it’s more legible on screens than Baskerville”, versus “Let’s use blue because it’s my favourite colour”, or “Let’s use Helvetica because I saw it being used on that other cool website”. So, poor design happens when too many of those decisions are poorly informed. To be informed is the opposite of being ignorant. Therefore ignorance leads to low quality design. This is why designers should be informed. So, go read a book!