One of the benefits of capturing Raw files and processing them in Lightroom is that I can easily create derivatives of those files as needed. For example, if I need to send some files to a publication as PSD files or if I want to post some of the images to my blog as JPEG files, I can quickly batch export the images from Lightroom. As soon as those files have been received (or posted or whatever), I can then throw away the exported derivatives because I have the original raw files to return to and can therefore quickly export any additional copies of the files at any time. Of course this workflow might not work for everyone, but I find it convenient that I no longer need to keep track of as many derivative files. One word of caution, however: if you export a number of files and then do additional work (retouch the files in Photoshop, for example) I would keep those retouched files (as well as the original RAW files) – but I would still delete any derivatives created from the retouched files.
And while we’re on the subject, I would strongly encourage photographers to keep their original raw files – because you never know when you might need those high quality originals. Plus, I have found that I have been able to significantly improve the quality of my older images as the technology improves (which is exactly what happened to me with my window seat images – refining them with Lightroom’s improved processing is enabling me to pull out more detail with less noise than I was able to 10 years ago).
Of course there are photographers who are going to disagree with me, and for their workflow, they may be absolutely correct – they may never need to return to the images that they are making today, so there might not be a need to keep them. It depends on the type of work that you do.