After all, the only way the game is fun for them and for me is if we play "fair". If the rules let you make something, it's fair. If it's just made up, then they might as well not play. After all, it's easy to beat them if you can arbitrarily make a monster AC 36.
I don't play D&D as a competition between myself and the players, so that definitely skews my perception a little bit.
In my game, the intent is for everyone to have fun, DM and players. It's a cooperative play style.
Now, in 4e, I simply say "It's a level 10 monster, it has appropriate defenses for it's level."
And in 3e, you absolutely positively cannot say, "It has appropriate defenses to challenge you guys."
Of course this demonstrates the true genius of 4e "design:" the explicit permission to ignore the rules. Which you can only get from 4e. But definitely not 3e.
Ignoring the rules when they exist <> strength of the system.
Ignoring the rules when they don't exist = strength of the system.
I know a lot of people must read a post like this and say "what the heck?"
Nooooo.....
but if you say that you're likely not playing with a lot of "system mastery" type players.
Ergo, the only possible solution is to move to a system that is explicit about the fact that there is no system to object to.
Let me put this in a way that is free of edition bias:
When your players say to you, "How did that ogre get a 23 AC?" the correct response (regardless of system) is, "I gave it an AC appropriate to challenge you all."
And when the players ask, "But how did you specifically arrive at 23 AC?" your answer (again, regardless of edition) should either be,
1) "Based on my experience with the system, 23 AC seemed right."
or
2) "I looked it up on a table."
#2 is not truly non-edition specific, as the complaint is that 3e contains no such table.
This oversight, of course, was rectified in 4e: If you lack the experience, now you have a table.