Seven Two Sevens and baby Nines don't have pushers. MD-80s got them after Douglas had to pop the shute during stall testing.

Well this is not the case, as I fly a tranport category aircraft with a T-tail and no stick pusher.

What do you fly?

To Push the Stick
 

It is my understanding that during development if it is found that the aircraft has certain "undesirable" characteristics (may require exceptional skill) from the pre stall buffet (if any) through the stall break (if any, Controls may reach the aft pitch stop) that a stall recognition system can be installed. These would include big red signs, stick shakers and stick pushers.
The undesirable characteristics would be a roll exceeding 20° (wings level stall). The stall itself cannot be violent or extreme or needing anything but "normal piloting skill". A stick force per 'g' reversal or a reduction in stick force is usually a no no.
So they can add say a stick shaker if there is little aerodynamic warning and a stick pusher to really prevent the stall and send the message to the pilot that a recovery should be initiated when the actual stall will be "exciting". Delta fins usually are used to during power on (1.5 time the power to maintain level flight with landing gear extended and flaps at approach setting Vsr1) stalls where there is not a clean enough break or sufficient downward pitching moment through the stall.

AC 25-7B
FAR 25 Subpart B - Flight

Stick pushers were a compromise. When psychologists discovered that the current generation of pilots was too rebellious to obey stall warnings, a head-knocker was designed, but OSHA vetoed that as too cruel. The banana reward-dispenser was too slow, and invited a secondary stall while the pilot ate. The throttle auto-firewall system caused too many unwanted go-arounds. Designers just ran out of good ideas. :D

For whatever reason, the Dash 8-300/400 series has a pusher, while the -200/100 does not.

Something "undesirable" must have happened during -300 certification.

And I hand flew my 1960's Sabreliner at FL410 with the yaw damper off, something you couldn't do in a POS Learjet either! And I didn't have to sit with my head tilted to one side! :D

Some FBW aircraft do not have them because the elevator will pitch down, regardless of column/sidestick input, in the event the aircraft approaches the critical angle of attack.

How's that working for Airbus?