Regardless of the OP's motivation, I too consider Plate Pass to be a scam. First, why should I be charged for 7 days' use of a service if I use it only once during the week? Second, when you do see the charge, it is not the generally reduced charge that gets charged with automatic toll payments, but rather the higher charge for out-of-band charges (i.e. when your license is scanned and you get a bill by mail). Third, there is no way to opt out of it in places where the plates are scanned instead of use of a transponder - when I tried this in Washington I found I could not do a temporary registration of my rental car with the state DOT which charges the tolls, because Plate Pass had already done that. As they have the transponder, there is no need to also cover billing by license scanning, and it seems to me the only reason to do that is to force a renter into using Plate Pass. Because some toll roads (such as the Lake Washington bridge in Seattle, or the Sumner Tunnel in Boston) have no option to pay on the spot.

It is not to convenience the driver. It is to make money.