As I've shown before, I have a rather heavily modified UI, and most of those modifications come from Pitbull.

The big green and blue bars are my own HP and mana bars. The rest of the party and raid are listed beneath that. Each person's target is off to their right.
Compare that to the default frames provided by Blizzard:

Besides being more condensed (admittedly, the Blizzard Raid frames are more condensed than their party frames), there are a couple of things that non-Blizzard frames show that Blizzard frames don't:
- Who has aggro — if a mob is targeting someone, the player's HP bar goes red.
- What everyone is targeting.
- Incoming heals — you can't see it in that picture, but when a healer starts casting a heal, I can see the effect of that heal on their target before they finish casting.
I don't have it set up, but I could also see frames for just the main tanks and/or healers, etc.
Those seem like such little things, but they are absolutely crucial. You may not need that last one unless you are a healer, but those first two apply to any class.
Let me put this bluntly:
If you don't have a unit frames addon, you aren't ready to raid.
Your unit frames don't have to be as ugly as mine. Have the pretty pictures, and textures, and borders, and glowy effects, and whatever else makes you happy. But get them.
Yeah, they take quite a bit of customization to get to a usable state. But here's the thing: Blizzard's frames simply do not provide enough information.
- Tanks, how do you know your healers don't have aggro?
- Healers, how do you know the DPS hasn't pulled aggro, or that someone else in the raid is about to land a heal a half-second before you, thus wasting your heal?
- DPS, how do you know you aren't the only one in your party/raid targeting a mob? Or that the mob you have targeted isn't about to be CCed by someone else? Or when you Feigned Death, Faded, Soul Shattered, or Ice Blocked, that the mob headed toward you didn't turn straight for a healer?
Running with the default Blizzard frames last night, I felt blind. I couldn't see any of the useful information that I need to be a productive member of the team. I'll be reinstalling Pitbull tonight.