Anonymous is interesting because it's new, but unlike new things, you can't just be a "me too" groupie, or copy their rhetoric (you can, but read on).
You mess around with this haphazardly and serious consequences lie in the end. This may advantage Anonymous, because they are probably going to get stupid people joining who don't see the full implications of their actions and then get shot down quickly as a result (of being stupid); you'll also get dedicated members that probably have a brain on their shoulders and a type of nature in tune with the central rhetoric (or whatever you call it). Let's not forget that those who are neutral to them will not join either (because it is illegal). That seems like a pretty good way to build up a virtual force since it weeds the weak and untrustworthy out by nature; ironically, many of these theoretical benefits occur naturally due to institutions established by those that seek to destroy them.
Spies may be a worry though, but the nature of their organizational structure/tools must be such that this factor is mitigated in some way, or else they would already be gone. I'm not saying I agree or disagree with what they've done, are doing, or will do, but simply that it is interesting to see just how complex this organization (or whichever noun they wish to use to describe themselves) is. This is not as open and shut as some seem to make it and I will keep a keen eye on them. Thanks Tomshardware for keeping me informed!