[randall]

I'd argue because it's really not that hard.

Prototyping a product, and implementing said prototype, are hallmarks of people with vision and determination. If you have an idea but can't execute, why bother?

Bare minimum reason: You understand how the product is created, and can empathize / call bullshit when something is delayed.

[trustfundbaby]

You can prototype a product using Axure RP, Powerpoint, Illustrator, Blasamiq or Mocking bird ... you don't have to spend time learning html, css, javascript and or PHP just to do that.

[jtheory]

I think this is where I'd set the requirement, rather than requiring writing code.

If they can make (and talk through) a mockup that demonstrates a grasp of the problems of real-life UI/UX design -- including the little details that normally just get hand-waved over, like "when & where exactly will we get all of the data that's required to build this screen?" -- that counts for quite a lot.

Writing code for someone who doesn't have the background has a huge learning curve -- if someone sets them down with the exact language(s), libraries, editors, references, etc. to use, it's not so bad, but someone who doesn't know the landscape will go down a lot of very time-consuming dead ends on their own, and the lessons they'll learn will very likely not be worth the time wasted (assuming they have no plans to change careers...).