The modern flat movement, on the other hand, is more just graphics designers making something that looks good and doesn't work well. I've put users through user studies on converted Android apps and they have a hell of a lot harder time figuring out they are supposed to tap an understated flat icon with no label in the top corner, the new action bar pattern, instead of a big glossy shadowed button at the bottom like earlier designs.
I've brought these numbers to Googlers before at conferences, but they just shrug and say users will get used to it. So even Google admits the current flat designs are testing poorly and that they just hope numbers will improve as users learn to look for the flat hidden icons.
Design based on function and performance removes emotions that contribute to the overall user experience. A purely engineering approach can lead to boring design, IMO.
I really don't understand the performance argument when we've been dealing with skeuo for some time. Of course, skeuo can be overdone, but in balance, it's what many users find useful.