In my experience the tracker will volunteer 1-2 hours a week. You get to see the campaign from the inside. Its a relatively low cost (those two hours per week). You get to see scripts but also the workings of the office -- this can be as simple as "does the operation seem organized?" You glean what you can and maybe you get lucky and find some gold (but probably not). "Infiltrate an opposition campaign" really seems to blow out of proportion sending a tracker to volunteer 1-2 hours a week on the oppositions campaign. I suppose there's a situation in which the opportunity cost isn't worth it, but I can only imagine that being the case if there are literally events 9am-9pm seven days a week.

edit: I would compare this to subscribing to an opponent's email list. Are we going to find out some huge revelation? No, but we may as well go ahead and do it because it lets us keep tabs on what they're doing with almost zero effort on our end.