It's $29 dollars per person. How does that come out as $4 a day? For a four day trip, 4 people, that is $29/day. Cheaper if you have fewer people or do more days, I realize, but I think that's crazy expensive for something that saves you a few minutes and a bit of walking. Security lines are never that much of a hold up.
Added to that, to make it worth your while, you have to park hop every day. As Lisa said- I think park hopping is something that really isn't as common as it would seem from the Dis.
Don't need to use something to see the value in it. You have to see it as something that would be worth it to use it in the first place! There are a lot of addons that I wouldn't use but understand why many people would. For this one, you're basically paying $29/person to get through security faster IF you parkhop. I just don't see how that makes sense to enough parkgoers to keep the program afloat.
ETA: also, it runs on a schedule, right, because it's a shared bus system. So if you wait a half hour to catch it, it costs you a half hour- it doesn't cut your time in half at all. Uber could whisk you to the next park and you'd be through security before that bus shows up.