It can be annoying because sometimes the title of a necro'd post gets you excited -- found the secret to farming rare pinks!! -- and you open it and it's from 6 months ago and the information is either already common knowledge or is obsolete. That little thrill you feel at discovering something new deflates and you groan as you realize you wasted your time opening the thread.

On the other hand, I find it just as annoying to have 6 threads on the same topic, especially when the topic has been answered several times -- often on the post right below the newest post. So yay for newbies who at least search...?