Methinks that overall defense should be somewhat higher, so that there'd be *some* rationale to have defense > 25.
Maybe giving "strong" unit for defense > 20 would mostly do the trick - but ain't possible currently !
Balance is an issue : nations shouldn't be "equal" in defense, as they have different strength. Some are Ok (Ulm), but some soo crappy that they need a boost (Arco- but my upcoming mod will take care of this too !
)
However another problem is that defence can't have orders, so can't be trusted ...