It seems to me that there are really two (related) issues here:
1. Keep or delete derivatives? Here I'm not really talking about derivatives such as PSD/TIF files which are created in the process of doing additional work in an external editior, but rather the (typically) Jpeg files created during an Export or Publish process. Obviously, as is the case with many workflow issues, there is no right or wrong way and we all have to work out what suits us best. In my own case I do keep some exported jpeg files, though now less that I used to. Prior to LR3 I would export sets of jpegs for uploading to the web (Flickr and Picasa), and generally I would keep those derivatives....though to be honest in retrospect I now wonder why! Today I use the Flickr Publish service (Lightroom's own, not JF's), and as Cletus pointed out no 'intermediary' jpeg is created but rather uploaded directly from Lightroom. However, I also make quite a lot of use of the Publish to Hard Drive facility....for creating jpegs to be synced to my iPhone, copied to a CF card for transfer to a couple of Digital Frames, and some are exported directly to my NAS drive so that I can force my family to look at them whenever they've misbehaved! So all of these hard drive Publish derivatives are currently retained (some because they need to be, some because I haven't bothered deleting them).
2. The second related issue concerns importing the jpegs back into Lightroom. Personally I don't see any value in doing that as all it would do is double the size of my catalog for no obvious benefit....I regard all such derivatives as truly disposable on the basis that I can easily recreate/republish them whenever and I'm never going to edit them...so I really don't need to have them in Lightroom. I keep originals and editable derivatives only in Lightroom.
Back to the OP's issue....I think there are times when it is perfectly understandable to keep copies of exported jpegs, though perhaps you might want to ask yourself what value you would get from adding the exports back into your Lightroom catalog. Not saying you won't have a good reason for adding them back in, I'm just saying that for me I can't find one.