Why the masks? At least in Britain, they wouldn't be necessary if it weren't for the presence of Forward Intelligence Teams (http://en.wikipedia.org/wiki/Forward_Intelligence_Team) who are deliberately used to scare people away from protest by overtly photographing those involved.
Got a public-facing job? Demonstrating against corporate interests who may be connected to your employer at some level? Thinking about ever applying for anything that requires security clearance at some point in the future? There are a lot of reasons why people should expect a right to anonymity in protest, and the Guy Fawkes mask is, in truth, a lot less scary than the alternative, which is a scarf around one's face. The scarf has long been demonised as being indicative of a desire to riot and those wearing it will often be singled out by the police; it carries connotations in a way that the mask (being uniform, slightly silly, and from a comic book/hollywood movie) doesn't.
It's practical, unifying, and less easy to demonise than the scarves-and-jackboots-blac-bloc look. What's to hate about it? Let them have their fun; once it gets widespread enough, the state'll just illegalise masks altogether (last time they tried was 2010) and then it will be beatdowns for anyone who has any expectation of a right to privacy. Yay!