As a personal and squad-based (my own squad at least) opinion, I amd all for both leaving aircraft in for the whole rotation once they are enabled, and for allowing aircraft in based on service dates rather than date its country entered the war. Both for similar reasons.
Our squad, being a Romanian 109 squad, historically flew the 109E3, E4, and E7 until they were replaced by the 109G2. For historical ops (for us) this would cause a problem if the 109E was removed when the 109F became enabled. For squads based in other theaters, this could also be accurate because just because an aircraft is available, doen't mean it had reached all fronts yet. Just look at the 14th AAF in China or the RAF in North Africa as an example.
Also, I support the inclusion of aircraft before their country was actively involved for two reasons- lend-lease and subs. Like the fact that the Soviets and RAF flew P-40s well before December 1941. And substitute wise, the A6M2 is a great sub for the early model IAR 80s we could fly as another ARR ride (even though our particular squad never flew them). I know we were considering making the Zero (IAR) and Ki61 (poor He112) our main rides in the event of a PTO planeset.
I know those are selfish causes, but I figured I had better explain my reasons for endorsement beyond a simple "I like it".