Just having a look into the calibre database. the Books table already has an LCCN column in it. (interestingly however, the column isn't listed as an option to switch on in the customize column preferences dialog).
I think anyone concerned that adding Dewey and or LCCN columns would somehow mangle the Calibre interface, are not considering the preferences capability that already exists within Calibre. It would be quite easy to simply switch these columns off if one had no use for them. It would also be the same to disable a plug-in that was put in place to look up this information. Hence no impact on either performance or look and feel.
While I think having an optional folding hierarchy based on Dewey would be ideal, its certainly not imo critical. Having the information on the other hand would be a great benefit.
