I love Matx and mini ITX.
I used to be a big supporter of large cases (even had a double wide watercooled 75 pound monster on casters), I liked big ones because I could get in and work on them without things in the way. But that stuff is no longer needed. We no longer need sound cards, network cards, 10 hard drives and 1 or 2 dvd burners.
I have 3 systems in the basement which are all relatively similar.
Attached is my watercooled, raid SSD, MATX setup from a while back. At the time it didn't have a modular power supply, but it still extremely barren and allows easy access to everything.
I love aluminum, but you pay more, they are less durable and they never match the breathing ability of a full mesh front and on a small case, airflow is more critical than in a large case. I start with a small MATX case with full mesh front (I like the Fractal Define Core 1000), then rip out the drive cages and put fans down the front. If it's watercooled, I stuff a 2x120mm radiator there as well. If the system needs hard disks, I drill mounting holes in the bottom, if it uses SSDs I use a a PCIE mounted drive bay (as seen in the attached picture). With or without a modular power supply, you end up with a very empty looking case that is very compact, easy to work on and no dead air spots.
I believe this was a PowerSpex case that I paid $25 for, the Fractal 1000 is actually shorter. I cut out the front of the upper drive bay so the radiator would slide up inside. The bay still could hold two dvd's if I ever repurpose it for something else. Even the old drive bays can be put back in with screws. Ignore the blue lights, they were what I had on hand. The weird connector out the back runs to a remote switch box on my desk as this sits about 5 feet under the office floor. The water cooling system originally was a Corsair AIO with a single 120mm radiator, I swapped that with a dual I had from my monster case, and made a short tube type reservoir in the drive bay which goes up to a fill port out the top of the case. It easily handles an overclocked I7 2600k.
Tp4tissue, I'm not sure where you get the idea they need as much space as ATX, the lazier designs do, but some are quite a bit smaller. I used to feel like you, that they were hard to work on, but look at the attached picture and tell me it's hard to work on, even with watercooling and a non-modular power supply. There's not even cables hidden behind the motherboard.