The only thing that will perpetually stay in skirmish is HA though
If I was unclear, I did not mean that any unit will "perpetually stay in skirmish."
The flank with "Inspired Defense" has its own worst-case-scenario: it can cause the second tactic to be fired to be "Inspired Defense," providing 24 days of this skirmish tactic.
The odds of getting the second tactic to be "Charge on Undefended Flank" are only 25%. But for the second or third tactic, it is 46.75%. And for the second or third or fourth tactic, it is 57.81%.
So there are 57.81% odds of getting to "Charge on Undefended Flank" before day 13 rolls around. When day 13 does roll around, the Inspired Defense option means one of two things:
13.5 weight for another "Inspired Defense" (if you are using a Defensive Leader with martial 14+)
or
4.5 weight for another "Inspired Defense" (if you are using an Inspiring Leader with martial 14+) and, once in melee, a chance of "Religious Fervor" (which is itself an inferior tactic)
That said, one would have to do the full calculation to see which is better. It is not obvious at first sight.
(In actual practice, it will all be very dependent on the commanders actually
available to you. So you are right that "one size fits all" doesn't apply in practice.)