It's to my understanding that you can simply go to jail to decrease major skills. I'm not sure what the max is, but the higher your level, the more overpowered NPCs and boss monsters are. Even daedric weapons and armor seem useless. Powerful spells do minimal damage. Because leveled equipment officially stop getting stronger at level 30, and how powerful your character is is proportional to how strong other NPCs are, it's probably smarter to stop leveling past 30, even lower than that if you specialize in a single fighting style.

I've managed to max out every attribute aside from personality and luck at mid-20s. Because increasing remaining attributes and maximum health (which is why it's important to max out endurance asap) are the only true benefits to leveling after level 30, what is the value to power leveling? I consider my odds less in my favor at higher levels because that means NPCs are catching up their attributes and will eventually be even with me. Also, what is the highest level character you have worked on?

Actually, in Oblivion few enemies scale with the player. Most enemies behave exactly as they did in Morrowind: i.e. they have permanently fixed stats and appear when the player reaches a certain level. As the player progresses past 30 he'll find that most enemies (Goblins, for instance) will get weaker relative to the player - while a few enemies (Goblin Warlords, for instance) remain within a level or two of the player.

But, to answer your question, I see no real benefit to power-leveling. My highest character was in her 40's and, combat-wise, I felt the game got bland by the time I hit level 30. Some people say combat gets harder as you level past 30 but that has not been my experience.

For many reasons similar to the ones you cite, I cap my characters (by picking some 'never use' major skills) at a predetermined level that will always be between 20 and 30, depending on the character. I also like to level slowly. My current character has about 600 hours, but achieved her top level of 20 at 400 hours.

Why power level? It's something to do. Most mmo's are full of power leveling I find character development is one of the more interesting aspects to some games. Gaining perks abilities or stats and equipment is usually the main point of advancement for most games, Elder Scrolls has a very rich story line though so power leveling doesn't seem essential to me but some games it is.
I find character advancement interesting because games tend to have a lot of constraints due to programming being so complex. My favorite things in game is fully destructible environments and parkour. When you get down to it those are the most basic things the games environment and how you get through it, destructible environments are kinda impossible to program which is why when you pick up a gun in a game and shoot the ground little circles appear and disappear after a while. The parkour thing how the characters move kinda sets the entire tone of the game most of them you move in a straight line without the ability to jump and most stuff is just an obstacle to keep you from going out of bounds instead of hurdle or springboard. So getting a whirlwind attack with a sword or +movement speed is usually the fun part for an hypothetical example.

What do you mean by "Power Level". That has about three meanings:

• Focus on increasing levels as quickly as possible, at the expense of role play, quests, etc
  
• Efficiently level, meaning getting 5/5/5 or 5/5/1 attribute bonuses at every level
  
• Increase your level to insanely high levels using the Jail Glitch

From your post, it sounds like you're talking about reaching level 200 or something like that. Yes, there is no point in that except doing it because you want to. You do get relatively weaker. Most creatures don't level. But NPCs and some creatures do. So it just ends up making things more difficult.