Well, otoh, the same DMG states that an adventuring day should consist of 4 encounters of EL = APL. So if you're going above that, by throwing more and/or higher-CRed encounters at them, you technically _are_ running the game harder.To deflect this, I usually point to the DMG, and how I typically follow the guidelines in it pretty much too the letter, with each adventuring day using up roughly ~80% of the party's resources.
Granted, most MMx monsters are little more than speedbumps for a well-built party. In my home group we usually handle about 2x to 3x of the expected allotment between rests. Which of course also results in a quicker leveling pace. And that doesn't even require obscure splatbook optimization - my personal record one day was starting the day as level 9 and going to bed as level 11, in a core-only campaign with no wands available.
So, in short, the CR system simply doesn't do what it's supposed to do.
That said, I once had a DM who seemed to run a similar style as you. He'd spent hours and hours on end designing single encounters, building opponents and devising tactics. And of course every single monster used its actions optimally all the time. We got our asses handed to us on a silver platter on a regular basis - about 2 in 5 encounters. And guess what? We were not having fun. We gave it a try for about 6 sessions or so but seized the first opportunity to rotate DMs and switch to something different.
What I learned from this experience is that it's super easy for a DM to frustrate the players. The DM knows exactly what the PCs can do, and that's usually just 1-3 tricks per pony. For every one scenario that a player can prepare for, there are probably three dozen more that he can't, so it's trivially easy to attack their weak spots any time you want to. After a while it just feels arbitrary.
So yeah, call it "big dumb fun" -- I guess many players (including myself) enjoy besting powerful opponents or overwhelming odds by virtue of their abilities and wits. They do not enjoy being bested by smug little buggers playing tricks on them.