The problem with using the quote box, with or without attribution, is that is gives the appearance that the quote was taken from some post verbatim by the software. This would be misleading for modified quotes.
Italics, CODE boxes, offsets and indentations, quotation marks, and other techniques do not give such an immediate perception that the quote is a direct copy. It is not that any form is utterly free of confusion, but keeping the QUOTE boxes sacrosanct for unmodified quotes imposes a certain consistency of application that provides a level of expectation for any reader at any time.