Look, here's my view on thread digging:
Someone who actually searches the forum before making a new thread posts something useful or a valid question in an old thread. Bam, that's fine with me.
Someone digs up an old thread for no reason ( p1ng digging up the old chat forum threads, kuja's #1 digging up the mantralord's sister thread) and just posts some random comment or asks a completely irrelevant question. Goddamn it, that's bad.
Basically, if the thread dig is in a forum other than the chat forum, it's more likely to not be spam.