Of the many complaints about 3.X that are made, often for very understandable reasons, this is one I understand the least.
You do not have to make certain that PC's have the correct "wealth by level". In 10+ years of DMing 3.X, I've never once calculated PC wealth or tried to calculate treasure based on expected wealth by level. Likewise, while I have on occasion gone through the step by step process of modifying a monster to achieve a custom result, no one is forced by the system to either modify monsters in the first place (as opposed to just playing them straight out of a Monster Manual or other resource) or use some system when creating a monster. Whatever you do as a DM is perfectly right and valid - assign hit points, skills, or whatever as you see fit. Or don't. Just write down the minimum you need to know to run an encounter, and fill in the details on the fly when and if you find you need them.
Sure, I can see that. But that is a burden you put on yourself, and not something imposed by the system. It's something of an open question if a system can actually impose anything on a GM, but what you are talking about is simply guidelines that exposed the underlying math, and not hard and fast rules that a GM has to abide by. (Seriously, is there such a thing as a hard and fast rule that GMs have to abide by?)
You could of course ignore wealth by level. However, that meant also tossing out CR as a tool for estimating encounter difficulty, since WBL was factored into that difficulty.
Coming from 2e, some of the DMs at my table felt that the expected treasure from WBL was Monty Haul, and therefore handed out significantly less treasure. As result, the party kept getting their butts kicked after the first few levels, and deaths were frequent. This encouraged the 5MW because we were so below par that we had to Nova hard to get through a single encounter, with maybe just enough to spare in case something attacked while we were resting. These DMs then complained about those habits.
I tried explaining to them what was happening, but they didn't listen. One DM went so far as to rig an encounter to "prove" his point that WBL was overpowered. He placed two hidden rogues in trees and spent the party's entire WBL budget on invisibility potions. He gave them surprise and annihilated the party. 2 members were dying before we could act. The other two could probably have run away but tried to save their companions, and were subsequently easily dispatched as well. He claimed this proved that WBL was overpowered. When I pointed out that he had set up this encounter specifically so that we had no chance, and that moreover the party wasn't likely to blow all of their treasure on a single encounter, he dismissed my analysis and told us it had all been a dream.
Shockingly, whenever I ran 3.x (using WBL) the party could typically get through multiple encounters. Eventually those DMs picked up from my example that that's how the game was meant to work. Death wasn't uncommon in my games, but the turnover wasn't nearly as high, and 5MW was less of an issue, although it could rear it's head if the dice were against the party.
Sure, if you ignore WBL and ignore the encounter guidelines and ignore the monster design rules you could run in a very 1e/2e style that was generally pretty good. I only knew one DM who ever mastered that method. It wasn't me. (Although in fairness I was relatively inexperienced as a DM back then.)
So no, I wouldn't call it a choice per se. I don't doubt that there were DMs like yourself who could pull it off, but not everyone had the skill to do so. For those who lacked the skill, it wasn't a choice. Sure, I could leave off tallying skill points or the like if I was feeling lazy. But especially for a caster, not picking low level spells and then using them at whim would have felt like cheating. If the players have to memorize their spells, then so should my NPCs. That's fair. I did occasionally leave off memorizing all their spells, but in those cases they were considered to have been expended earlier in the day.
3.x was exhausting for me to run (and even to a lesser extent to play, as I had some characters that required a spreadsheet). It was not an easy game to run RAW.
