(which requires us to memorize a freakin' poem to remember how long months are)
It's a system that affords an easy division of the year into "quarters" (12 is highly composite, contrasted with prime 13), based on tropical seasons of unequal lengths (thanks to our elliptical orbit), seasons on which agriculture (the basis of all civilization) depends. By gaining ease of use in one area you're losing it in another.
In other words, we need to accept the fiction of an inaccurate placement of bissextile days which will ultimately cause the calendar to drift, because we need a simple system.
When asked if AD 4000 should be a bissextile or common year, the best that modern astronomy in 2014 can say is "maybe." Changing the intercalation cycle from one based on 400 years to one based on 128 years does not add appreciable precision or meaningful longevity into the calendar, but does increase the complexity in implementing it.
Don't think Clavius didn't know that 400 years was less precise than other options.
In case you haven't noticed, you are offering completely inconsistent justifications, using whatever logic is necessary to maintain the status quo.
Even if I am, the status quo is already in place and has a lot of social inertia behind it. To paraphrase Carl Sagan, the extraordinary change of implementing this calendar will require extraordinary justification. All I'm seeing here is a different arrangement of trade-offs, and we'd have nothing to gain from changing but entropy.
By the way, I'm pretty sure your billing software would work just as easily with 13 standard month lengths, and a day grafted on (as a fiction) to a neighboring month for billing or whatever
Which neighboring month? What's to stop the water company from appending it to month 1 and the power company from appending it to month 13? Hell, it only took 1800 years for everyone to agree that the new year started in January rather than March.
And if/when we do ever reach an agreed-upon standard for which month to append the epagomenal day to, why then treat it as its own month to begin with?
Again: epagomenal days aren't new, but aren't popular and are currently only used in religious practice (where "God Himself told them to"). I don't see how this new proposal addresses this old problem.
just as we already do with leap day
Because everyone agrees the intercalary day is a part of February, because that's a part of the defined standard.
And the reason the Julian calendar got screwed up has to do with an ambiguity in Roman counting about whether to count "inclusively" (i.e., including the starting and ending points) or not. We don't have that problem nowadays
Again: do arrays start with 0 or 1?
Regardless, it highlights the need for ease-of-use when communicating a concept to the general public, especially when transitioning between counting systems, which this proposal with its "month 0" does.
We don't have that problem nowadays, and with modern technology and advance notice, it would be easy to implement non-leap-day years whenever necessary to be more accurate
We already have that with the Chinese calendar. The PRC's national observatory communicates exactly when the first lunar conjunction east of ecliptic longitude 300 degrees occurs (Beijing Time) and the duration of every true lunation thereafter (also Beijing Time) until the next new year. It is as precise as modern astronomy allows, because that's exactly what it is. Why aren't you using it?
The real reason for many of your quibbles is simply because we have a standard time system and nobody wants to change it.
... and?