I can imagine this happening more often if people have to carry more passes to minimize the tolls they pay. I was tempted to consolidate by adding a tag to my brother's MD account since he lives there, but decided against it.
It will be extremely minimal the number of people that have multiple EZ Passes to try to get the absolute best rate at all times. Most people aren't going to spend the time to look up each individual agency to get a tag, then tie up their money with that agency on the rare occasion they're using that toll road, then have their EZ Passes placed in their vehicle so they can switch them based on their travel plans. There's a few that do, especially in the AARoads family, but it's rare.
Very few people use multiple agencies in a single day. Maybe someone commuting from NJ may benefit due to all the surrounding different tolling authorities, but NJ's EZ Pass program has agreements with nearby agencies for discounts, which greatly eliminate the need for multiple EZ Passes.
MD offers a discount for their tolling points, but you have to have an MD address to qualify. And people in MD using their EZ Pass probably aren't traveling thru other toll plazas too often in other states where they would benefit from another program.
If people want to save money, there are a lot of other ways to do it. Having an app for each convenience store and restaurant they visit usually results in savings on a per-visit or accumulated point basis. Clipping coupons. Searching out deals on 3rd party websites. Knowing the gas price at every gas station they pass to hit the lowest one. Driving slower on highways to increase fuel economy. These are all everyday ways to save money, rather than saving a dollar or two on a rarely used EZ Pass facility.