I understand the dislike of top cases considering it adds to the drag and top-heavy weight of the bike esp. since a full bag of camera gear can range from 15-30lbs. That said, there are some obvious benefits to a topcase in terms of it's use for photography and photographic equipment.
Consider, for a moment, the trees. Just kidding.
One nice feature of a top case is that it's at a natural working level for a standing human body (unless you happen to be 5 feet tall that is). Also, I've found that it helps when changing lenses in dusty/drizzling environments (unless you're one of those super human photographers that only uses 18-300 wonder lenses so you never have to change lenses and think there's nothing wrong with a 2,000 dollar body that only produces soft images).
Then lets not forget that it's easier to balance out other gear (camping, tools repair, clothes, whatever) on the left and right side cases than it is your photo gear. You're not likely going to put your 70-200/2.8 in the left case then second body on the right. Even though a top case puts the weight up high, it is laterally evenly distributed.
Water. You may not plan on crossing water but if you are there's NOTHING like having the case as high as possible which also has other benefits. As long as the top case isn't huge, it's often the last thing to touch the ground in an impact whereas side cases are often the first. Maybe I'm nuts but should I low side in the mountains on a wet road I'd rather have my tent and sleeping bag taking the hit inside the case than several thousand dollars of electronics.
I'm not trying to say a top case is the ONLY way to put cam gear on a bike but really don't understand the attraction to having it in a side case?
Please let me know what I'm overlooking...