A corollary to the limits of anti-spam precautions
One of the corollaries to the limits of certain sorts of anti-spam heuristics is that if you see spam software actively working to get around any particular sort of precaution, you can assume that the particular precaution is getting reasonably popular.
This follows from spammers being lazy but not stupid; if the precaution was not reasonably popular, dealing with it probably wouldn't be worth the time of the programmers behind the spam bots. That they do deal with it implies that it is worth their while to do so, which suggests that it is reasonably popular. The harder the precaution is to deal with, the more this holds.
(My strong assumption these days is that essentially all spam software is being written for cold-blooded commercial reasons and so its programmers write the features that will make them the most extra money.)
Of course, all bets are off if you annoy the programmer of some spam software. But this probably isn't very likely for most people.