Apple’s guidelines have always said that developers should avoid splash screens ‘or other startup experience.’ That's nothing new, but many developers ignore it anyway.
For much larger apps the "fake screenshot" is harmful. Take the Facebook app for example - what should the "fake screenshot" be? iOS does not differentiate between a completely fresh launch vs. a simple restore from background, and will show the same image no matter what.
So now you're in a situation where you've presented your users with a fake/blank Facebook stream, but really they were restoring to a photo they were looking at. Oops.
Or hell, do you even know if your user's logged in? Would be a shitty experience to show them the fake/blank stream but suddenly pop them back to the signup/login page no? What if they are logged in? Would be a shitty experience to show them the signup/login page and suddenly yank that out from under them.
This whole business is a shitty solution to a shitty problem: apps take forever and a day to launch.