I don't have much of an opinion on them. While they have stopped bad things from happening they've also caused bad things, too. They have existed for so long it's hard to imagine how the world would have turned out without em'. Night Watch seems to lean a little towards the pro-side. ie "the 'bad guys' already have the weapons so the 'good(er) guys' need them, too.
Men at Arms is a little different in that this is the CREATION of a weapon, not a continuation of one. No other people own this weapon so if the message is 'these are bad' then it's a little weakened by this. I mean, we can't exactly go back in time and stop the creation of a weapn. Maybe, and this is a big maybe, this story is just a "what if we COULD stop this from being made. If we COULD, maybe we SHOULD" type of thing.
It might be a while before I even get to this one, though. I've been meaning to read this other non-Pratchett book for a while.