You'd be supprised just how strong a toughend screen is, especially when it is weight pressing near to the supported edge. I'd happily carry 2 bikes on it. The curve of the screen increases it's ability to support the weight. If it were flat I would think twice about carring 2 bikes (but still would [IMG]/img/vbsmilies/smilies/smile.gif[/IMG]). The angle of the window means that increasing the weight on the rack tends to make it slide DOWN the window rather than pressing inwards against the class so there's not as much load on the class as you would think.
I have worked on buses where it has been necessary to bend one of the side windows out of the rubber seal. We would use a strong piece of wood (hammer shaft) & force it 2" out of the frame while 1/2 to 2/3 of it's length (approx. 1 meter) remained fully in the seal. 2" bend over 12"-16" of it's length meant none of the bus mechanics would come near us. Never broke one in the 40ish we did.
I have also hit one HARD with a hammer (by accident) without even a chip. It's not the force/weight that does the damage but the area of the impact. Small and/or pointed being the worst.
To be honest I would worry more about causing damage with the lower end on the bodywork.
The 3 bike rack is a longer unit & therefore more suited for high level whereas mine was originally a low level rack designed for the lower bar to rest on the bodywork (between window & Nissan badged trim) but that way would require a light board. I'm not sure if the tailgate can be opened with the 3 bike rack, as you say, they are designed to rest on the bumper.
Airsoftaddict will have to make up his own mind based on the number of bikes he intends to carry as well as type of car he has and the rack he gets.