I technically do not have layers cut into my hair BUT my hair does not sit at all one length. I've had it cut, when wet and straight, all at the same length for years. Now, it may sit all the same length when wet and straight, but when it curls it sits at different lengths. If you think about it, the underneath layer of hair sits a lot lower down my head and therefore is a lot shorter than the top layer. So, being curly, it springs up a lot more. For this reason, I have an 'undercut' unintentionally. It does make my hair look less full and whilst all the top layer looks the same length, underneath I have much shorter ringlets.
I am trying to reverse this, and gradually let the bottom layer grow longer than the top layer so I CAN get layers but it'll take a while.
So I cant say that 'all the same length' will work for everyone - a lot of people get 'triangle head' but as my hairs fine it sits flatter.