1. GNW, while there is white list you can still see your own exact position, which given knowledge of how the group usually works might give something away. This can be compounded if there are multiple spies. Furthermore, they would be able to tell exactly which top powers of the group are on, while this can be done via friend list examination, it might not always be possible and certainly would be harder that way.
2. Plunder, in addition to self plunder, you could also give away info on when others are convoying and have the main or others in your real group plunder them. You might even be able to, from chat etc, know who is supporting the convoy and therefore pass forward that information as well.
3. I can personally do 12% damage to my group summon in one hit, I'm sure there are people who can do more, but the point is that I'm ftp. While usually spy won't grow as strong as me, they CAN still do significant damage. When the point of group summon is to get as many different people tapping it as possible, usually holding off killing until the last 5-10 minute, spies who triple tap to kill it early can be disruptive.
Those are just what's off the top of my head.
Since the introduction of white list, spying is not nearly as bad as it used to be. That doesn't mean, however, it can't negatively impact the group at all. Thou I have to say a lot of the current sentiment came from the old days when spying literally cost you wars.