Right now, my feeling that the requirement for 'Layers' is only really needed to meet the desires of those who have spent a lot of time processing in PS. I don't think that PI would provide any significant advantage over what we are already used to if we had layers functionality available.
As Carlos stated, let's say that you have an image that has had a 'layer-cake' of processes applied to it. Just because you can then go back and 'tweak' one of the intermediate processes does NOT mean that the subsequent processes remain valid without their own parameters needing to also be 'tweaked'. OK, so there may be trivial cases where this statement does not apply, and where you COULD tweak a mid-stream process without needing to re-evaluate all subsequent steps, but I reckon that the adjustment could just as easily be made as a 'next step', rather than a 'layer tweak'.
History Explorer is our current 'equivalent' of the PS 'layer-cake' - the difference being that you have FAR GREATER control of the 'layering' process, the only penalty being the lack of pseudo-real-time response seen in the integer bitmap world of PS.
Even without 'layers', many of the processes available in PI are just too computationally intensive to integrate into a 'real-time tweakability' layer-cake preview - and that is true even for multi-processor power machines, with plenty RAM on tap as well. So, users would have to confine themselves to VERY SMALL image sections in order to get a dynamic response from their layer tweak, and such small previews will often NOT capture the feel of how the OVERALL image response might look like.
The lack of Layers in PI simply enforces a paradigm shift in processing methods compared to PS - and so what? Just about everything in PI forces the same shift anyway, so what is the problem? Even if 'Layers' become available in PI (and, if I were Juan, I wouldn't waste ANY TIME WHATSOEVER developing this at this present moment), the PixInsight implementation will NOT be a 'mirror' of that used in PS - simply because the PS implementation is very 'crude', again because it was just NOT designed for the floating-point, high-dynamic range world of astroimages.
So, by all means let's add 'Layers' to the PI 'To Do' list, but if that particular Post-It note happens to fall off Juan's monitor and gets eaten by his dog, I don't think any of us should be overly concerned. As Sander says, there is a far greater requirement for full documentation to be created, long before something trivial like layers.
Cheers,