Becase the NEF files are a single channel of data, while the PSD is demosaiced into 3 RGB channels.
Plus PSD has expanded the metadata from binary form to text form in several places.
PSD is compressed, but not as aggressively as TIFF files can be.
Once I've made the initial adjustments--cropping, color correction etc. I don't feel a need to ever go back and never do.
There may still be information in the RAW files that might prove useful, especially if you should be working in 8bit.
I don't feel a need to ever go back and never do.
In the future you might improve your workflow and understanding of image editing issues and could possibly reap some benefit from not destroying the original information now.
Good point made c.pfaffenbichler however, my thinking is this--there is time spent on the raw file and then there is much more time spent on (usually a psd) the file once in Photoshop. For me to then go back to the orignal raw file, after having worked on it on PS would mean getting rid of all the work (larger amount of work, time wise and artistic wise) done on PS which seems pointless. Although the psd file does show your layers and stuff it only shows the end results of that layer. It does not show from where to where you pointed your brush, from what point to what point you changed the color or part of an image etc. etc.Anyhow I understand why most people keep their raw files, but this is the main reason why I do not. It would mean hours of work on an image you already worked on (and usually were satisfied with) to perhaps make some minor alteration. Also please note that though I was noce a pro photog, no I do it mostly for fun. Getting the exact red in my Coca Cola can has never been of importance. On the other hand, if there were a way of working on a raw file within Photoshop and keep it (save it as) a raw file equivalent, then I would absolutely do so.
You are certainly right that after certain operations (touch-up, transformations, painting, elaborate masking/montages …) the original RAW content may be close to impossible to re-introduce into a montage in an efficient manner.
I like to keep the RAW content in the layered file as a Smart Object in the first place and with Liquify now working on SOs that can be useful … but still once additional touch-up etc. has been done in Photoshop the SO becomes not especially useful admittedly.
Keeping the RAW files might be useful for archival purposes (like later on determining the date when an image was taken), but naturally that may be of no importance to you.
I'd say that keeping the raw file is paramount, as demosaicing improvements might allow you to recover images in a better way, reduce their noise, etc.
Here is a PDF white paper, by the regretted Bruce Fraser, explaining raw capture and why it is larger as a PSD than as a NEF file: http://wwwimages.adobe.com/www.adobe.com/content/dam/Adobe/en/products/photoshop/pdfs/unde rstanding_digitalrawcapture.pdf
(BTW, I find the "DAM" in the laboriously long URL containing 3 times adobe quite ironic...)