The very nature of drive tray usually means in modern cases they are mounted directly behind the main intake fan or have their own fan in some other cases, this is always a vast improvement over the old standard layout where drives were mounted directly below the 5.25 bays and recieve no airflow whatsoever, in 90% of home systems that still is the layout and most people have no problems, so its nothing to worry about.
Im also training to be a scientist and engineer i could design you a HSF i know how they work better than most i can even run the calculations(not that i want to they are really evil), dont be worried about your hard drives when it comes to heat they are designed to operate in most PC environments you only have to take extra measures if the environment the PC is in is rather more hostile than is usual.
Most cases for PC are very poorly designed and the airflow is not good.
There is a difference between a device surrounded by air and one enclosed which limits airflow.
I am not sure of how you have come to the conclusion that most users have no problems. I know of people who have had failures of disks and other items in the computer. Heat does not mean that something is going to fail instantly. It causes damage and there is an effect over time. Such an example is a computer room which overheats and then watch the failure rate for several months afterwards. Been there, done that.
Not all drive trays are created equal and it IS important to think about, especially if the drive is going to get heavy use. There are many design aspects to a disk and while you can make a generalization, it does not mean that it holds for each specific items, but what can be said is that reducing the heat will help to keep the item running longer than otherwise. You might also want to consider that some disks are designed for harder use than others. In the PC market, cost is an issue and most are not constantly used, which can make heat more of an issue.
I personally feel that you are performing a dis-service when you tell people to not worry about heat issues. Unless and until you look at what the specific disk was designed for and then check to see what the actual operating environment is, you can not say if it is exceeding the specs for the device.