I agree that in most menus you know in advance the pixel density you need. However, if we are talking high quality UI, on par with what you’d get from Scaleform or similar, image scaling can and will be animated. The whole menu can fade-in from a small size to fill the screen, as is my case. Without mipmaps you’ll have visible resolution pops during animation, or filtering issues while your image is less than half the intended final size.
I also have a case where some UI elements show up at different sizes because you can zoom them. If I have 3 zoom levels, and have to support the whole PC resolution range, I’m going to consume more memory by having each one of the texture sizes than by using mipmaps.