Wait i think I have severely misunderstood what the add on does. Are you saying it lets you know when any enemies are nearby no matter what?In some scenarios it'll help you avoid getting ganked, but more often that not it will just make it easier for groups looking to gank people to find targets.
You're right that wpvp is mostly an unbalanced mess, where the biggest groups win. One of the few ways of overcoming odds is if you are able to catch a bigger group by surprise by coming from a direction they don't expect or at an opportune moment. Some will obviously see your group coming, but if you catch enough by surprise you can change the odds by taking out enough people early on. Thanks to Spy, feats like this will be near impossible because in all likelihood, the massive majority of the group you are trying to catch by surprise will all be notified that a certain amount of players of the opposing faction are entering their proximity, and they'll be able to target them as well by simply clicking on the list that has appeared in their addon.
I thought this add on only showed when enemies went in and out of stealth.