So you can remove many posts without let posters knowing? This rule is a bit weird, first time to see.
I would like to see some justification (if any!) for reasons to which you should reply to a spam megathread (I should've copyrighted that). It's just not constructive, full stop.
The replies that most spammers would post in those threads involve 'agree posts' where they simply quote a previous reply (which is highly likely to also be spam) and then agree with the statement, sometimes changing a few words but mostly recycling it or alternatively adding one or two useless points to it... they also involve 'recycled content' where it can be seen that the post includes just as much (or even less) information as posts before it, and is thus pointless.
There are also sometimes posts that serve as a 'question' which is usually vague and unnecessary but they don't always take place in megathreads.
Signature managers are the deciding factor for post quality - their stance at the end of it is final. Static.