Wire guard would be much cheaper to produce than stamped steel and provide less protection... if any. I can throw a baseball at my Trane and do no damage, doing the same on my old my old bryant would leave a nice baseball size dent in the fins.
That being said, the Tranes require more protection because the spin fin coils bend more easily and are more difficult to straighten back out. A wire guard isn't really an option.
The main difference I've found outside of the obvious protection issue is that the louvers seem to promote impaction of dust, dirt and debris apparently because of the increased velocity of intake air through the louver.
A good HVAC tech knows how, an educated HVAC tech knows why!