Think about it; games play more fun with balance. I'll bring a load of examples to the stand just to represent my point:

In world of warcraft-burning crusade, [yes totally different], rogues are able to kill everyone, except for healers and warriors. And even so, they can beat some healers. This is imbalance if you ask me, and this is just stupid if you ask me. It's annoying to be on the losing side because of stupid inbalances, and there are very few counters, making this such a "great class".

In Diablo II, some characters do rediculously high damage and can finish off other players in a matter of hits, while other characters are so easily countered, such as the sorceress as it is easy to pocess a high resistance to her specialty. Furthermore, characters such as the assassin appear very quirky and fun, but in reality are really underpowered, in terms of skills, and their horrid equipment. This is another inbalance, again, annoying.

In games such as WC3, certain maps have been realtered about 200 times just to perfect the balance, and each time, it's clear that the map is getting better.

Moving in, levels such as BBlair, red team usually always wins, because they:
A) Have a carrot directly on their base, making it easy to RTS etc.
B) Have two good powerups on the opposing side, which can be obtained while attacking, whilst on the other side there's a meagre EB powerup.
C) Some of their start positions place you almost on the other base, preventing some crucial scores for the other team.
Personally, I think skewing balance can annoy gameplay, and is therefore stupid. Some people like it when they're on the dominating side of it, but personally, that shows no skill. It just shows you're able to take advantage of flaws in levels, and that you were lucky enough to be on the side with the advantage.

In terms of different powerups being on both sides, in JJ2, in most levels it's justifiable due to the abilities of the powerup and the structure of the level. For instance, in my level [I'm only using this example because I know how I made this clearly], Starlit Summit, there's a bouncer powerup on one side, and RF on the other side. Now, you can easily attack from above using the bouncer powerup, and that can allow you to dominate the game if you do so correctly, as the level is quite large and easy to come in from above. Furthermore, they are empowered by fast fires. Since there's no seeker powerup, there isn't a completely equal counter. However, on the other side, there's RFs. These are good for chasing in the various corridors, and also are effective near the bases, as the RF impact is very annoying and can blast other players into pits, with ease. In my opinion, the two powerups on opposing sides allows balance here. This causes the symmetrical level to be balanced.

However, a powerup balance that I have seen before but to be honest is never pulled off correctly is the Seeker vs Blaster combination. I think the general argument is that seekers... well you know what goes here. And then blasters, never run out, making the powerup everlasting [till you die]. However, this is pointless in many levels without fastfires, as fastfires are the only things that power up blasters enough to make them match to seekers. And even then, it's still arguable if it's equal. Also, blasters will require you to gather the fastfires too, wasting extra time and therefore making further inbalance.

Now I'm gonna bring up some other examples of inbalanced levels. One of my first levels, TCS, RagCTF05 or whatever. It takes 4 seconds to get from one base, to the other, but the other way round, it takes 5. While you can argue it is only a second, but this effectively means that one team is 25% faster than the other team. To recompensate, I put the bouncer powerup near the base, but in reality, it's actually equally hard to access it from this base without sufficient ammo, as it needs a good jump with RFs, or an entry from above. This is another type of inbalance I hate.

Another one? Semi. Haha. Right, the carrot. Yeah, it's in the middle. Yeah, its attackable from both sides. Say you're going from your base to the carrot though. Here's an overview of what happens from red base: You'll go down the hole, and go straight to the carrot. Now here's what happens from the blue base: You'll fall down, and then have to fit through some annoyingly small slits in the floor. This can waste a lot of time, unless you have perfected the annoying fall. Then, you can run to the carrot. Is it major? No, but it's enough to make someone lose, even if they're better than someone else. You can argue that you should have perfected the fall, but why does the other side not have to do it?

BBSwing! What happens here? Well, one team [on the right] will have a carrot in clear view near their base. On top of this, they have another carrot towards the middle of the level. The other team however, only has quick access to the middle carrot. This carrot isn't even on view, and is equally close to the other base. This means, that one team has two options, while the other does not. You can argue, that to balance, there is a seeker powerup closer to the left base, but every player wants the seeker powerup, while players usually leave carrots only to flag holders. This still gives an advantage to the team on the right in my opinion.

If you do not see the point I am trying to get across, levels should be very close to balanced, or perfectly balanced, otherwise, they are annoying to play in my eyes. Quite a load of levels have achieved this, sadly few of the ones that are commonly played. I hate the thought of "if we were on the other team, I woulda nailed these guys", because it just shows one side is lucky. You may not agree, but that's my view.