I think humidity should be fine as the ends and top are wire. I can also understand why they may have done this, given it appears to be in an outside location. I am guessing the pexi-glass in on the least sheltered side of the cage. Not sure how well it would work though given that heat would disperse upwards and you could still potentially get air moving through the cage end to end.
Personally, I think the cage looks good until you start thinking cage dynamics. Without permanently fixing perches to the back wall they can only go length ways. Which means birds can't really fly from perch to perch despite it being a nice length. Alsso not much opportunity to provide vairety. On the good side you can have fun with hanging toy and perches and this is what I would probably end up filling a cage of this type with.
Cress ... Dandelion ... Tyde
a selection from the milipidi flock