The main difference I see, and this is speculation.
You've got 2 groups of planes. Turners and runners.
The Spit 9 and Niki are the fastest of the turners. The Spit 5 isn't to much slower, and turns significantly better than both. This is the main reason why the Spit 5 (& Seafire) have had such a surge in use in the past 6 months or so.
What attributes do you look for in a fighter?
Most people look for something like the following -
Good acceleration (the quicker the better)
Good firepower (obviously, the more the better)
Good turning
OR
High top speed
(or both, in the La7)
If you are already in a slow turner, you may as well max out your "turning ability" vis a vis the other turners, since you aren't going to out run the runners no matter which turner you fly.
In the case of a "tie break" in performance, firepower is usually the deciding factor.
So if we added 2 more Hizookas to the Spit 5, here is what your average MA player would see.
Spit 9 vs Spit 5.
Turning - advantage Spit 5.
Acceleration - very small advantage Spit 9.
Firepower - advantage Spit 5.
Result, take off in a Spit 5.
Niki vs Spit 5
Turning - Spit 5
Acceleration - Not completely sure, probably very similar
Firepower - Spit 5.
Whats a Niki? (Granted, this would jive with pre-game knowledge)
Hurricane vs Spit 5
Turning - Hurricane (though not by much, in my opinion)
Acceleration - Spit 5
Firepower - identical (odds are the Spit would have more ammo)
I actually think the Hurricane would see a lot more use than the Spit 9 and Niki if the Spit 5 was upgunned, simply because it represents an even more extreme option for maxxing out turn performance at the expense of well, everything else.
The Zeke doesn't see any use simply because the added turn performance is counterbalanced by the poor high speed handling, and weak firepower (relative to Hizookas anyway)