The peculiarity about Strike Witches is that the series is basically just a show with the sole purpose of catering to Mecha Musume enthusiasts.
Now, propeller legs, lack of pants and animal tails and ears have been a recurring trait of them for years, albeit being thrown in by artists just to confer the girls a more moe vibe. Without no pantsu, there wouldn't simply have been SW in the first place. Simple as that.
As for the plot itself, it's no secret that it's just used as a means to an end, fanservice in this case. The whole job of putting together the plot was probably how to work all of these aforementioned Mecha Musume traits into the story in order to give them a somehow reasonably logical explanation.
Whoever is expecting something above a no-brainer is looking in the wrong direction.
tl;dr: It's of no use of looking too deep into SW to shed light on some questionable choices. It's part of the Mecha Musume archetype, so it has to be in SW as well.