I noticed that quite a few cases these days don't have the 5.25" drive bay or any drive bays at all.
The problem is that for optimal air flow through the CPU cooler, you want an unobstructed path between a front intake fan and the rear exhaust fan. Even a single drive bay will offset the top front fan from the CPU/rear fan and cause turbulence which reduces the effectiveness of the CPU cooler.
Better flow means more cooling for the same fan speed, or the same cooling with a lower fan speed. In fact, just having an minmally impeded flow can push mesh fronted cases into a region where they are quieter than heavily baffled acoustic 'optimised' cases which have to run their fans faster to for enough air through their more restricted vents. A mesh fronted case means you never have to do the 'take the front off to keep it cool' trick.
One of the things I like about the Lancool 2 mesh is that even though it doesn't have external 5.25" drive bays, it does have three 3.5" (hot swap, if you buy the backplane) drive bays behind a flip down side door. This is a great compromise.
Incidentally, I can't remember the last time I actually needed to use an optical drive. Even if I did, I would be better off with an external drive on my desk, which is easy to get to, than an integrated drive hidden away under my desk.