No, it really isn't. It's a way to provide consistent damage over a period of time. Every class in the game is effectively limited in a resource to do their job with - rogues are time-limited by energy, caster classes (and hunters) are limited by a finite resource pool (mana), and warriors are mostly limited by their cooldowns. Generally speaking, they're good systems.

Would you prefer that rogues' energy pile be like mana, where we start with a finite amount and have no real in-combat regen of it? It'd be incredibly broken in PvP (Backstab! Backstab! Backstab! Backstab! Backstab! Backstab! dead in 10 seconds) so we'd have to have cooldowns slapped on all of our abilities, at which point what's the real difference between what we have and mana like mages?

Suffers from being harder to scale in practice though.
Casters get increased mana pools as progression, warriors get faster rage generation until saturation.

Nothing aside from a few gimmicks increases energy regen.
That means scaling comes purely from itemisation and skill upgrade.

Paradoxically you'd think that would make it easier to balance out as you scale up, but that's not been the case to my eyes relative to other classes.

'Hey, lets make this spell do twice as much for twice the cost. That's got to be balanced right?...'

Increased mana pool doesn't scale DPS. Sorry. It doesn't. All it does is scale time you can stay in the fight, something energy is great for. Caster DPS increases come from... itemization (+damage, +crit) and skill upgrades. Sound familiar? Energy scales fine.

Warriors are somewhat broken, and no one pointing to warriors and saying "Make me like them" should be taken seriously.

This raises the question, though: Do people with faster hardware have a quantifiable energy regen advantage? That would be interesting.

I was seeing, on my hardware (Athlon X2 3800+, Geforce 7800GT, 2GB DDR, etc etc), a +21 tick at a very regular 10-13 tick interval (and this includes Relentless refunds, so I expect that the number if possibly even less flaky - around 11 would be my guess), so every 22 sec, I'm regenning 221 energy compared to the expected 220.

I fired up a torrent in the middle of my 26-minute test, which lagged my actions to hell and back, and didn't seem to affect regen, including the odd 21-tick. As we've seen, people have experienced 19-ticks, as well. I did recently switch away from Discord to an Ace-based UI, so I have significantly less UI lag, and I'm running pretty smoothly right now. I'd like to see my mod run on a wide variety of UI and hardware setups, so we can see what energy regen looks like for different profiles.

I think it would be...very interesting, at the least, for energy to be affected by client hardware. That would lead to a whole new level of min-maxing.

[Derail]
Curious as to what your perspective on the broken rage system is :)

Xtee: It's a sigmoid curve with gear, instead of roughly linear like everyone else in the game. It's broken as all hell during the fast ramp up phase, which seems to be roughly mid-late BWL (end of rage starvation) through early Naxx (everyone else catches up due to rage saturation). After that point, or before that point, it's fine. In between, they scale far, far better than they deserve.

It's an interesting mechanic, one I really like as a mathematical abstract, but in terms of balance I have issues with it. If they flattened the curve a bit (reduced the ramp-up in the fast increase portion of the sigmoid) and retuned the slow ramp portions it'd be a lot more tolerable.