I think you're very sensible to be questioning this.
When the weather over here (UK) improves, I'll be making a split and moving one colony into a Traditional Japanese 'Nest of Boxes' fixed-comb hive, enlarged to the same dimensions as the Warre. The main difference between this hive and the Warre is that the combs in the Japanese hive run uninterrupted from top to bottom (as they would in a feral hive), and as such need to be supported by spales (sticks running across the boxes).
I've heard that two others who have tried such a hybrid have run into problems with comb collapse during peak summer. Not surprisingly, this is causing me some concern. Ok, so Britain is not exactly famous for it's hot weather, but every so often we do have 'rogue days' when it becomes hot enough to melt the tarmac on the roads.
Although the combs in a Warre hive are supported in each box, Warre (in 'Beekeeping for All') recognises the potential for over-heating and makes recommendations to keep the hive in the shade, and open-up the top to enhance ventilation on hot days.
Perhaps the best source of info on running a Warre is at:http://uk.groups.yahoo.com/group/warrebeekeeping
where I recently aired my concerns about the risk of comb collapse. Unfortunately they seemed to interpret what I thought were valid concerns as being unwarranted criticism of the Warre design.