I don't object at all to this change, but I wanted to caution those using custom_json's for derived consensus data, that this may change how you interpret ordering.
You can no longer do async/parallel application of transactions within a block as the account would not be guaranteed unique.
In other words, right now one can shortcut ordering as blocks are clearly totally ordered. With this change, one needs to pay attention to the order of transactions within a block to ensure deterministic application across observers.