Depends on the situation.
If you're running as part of a team, mutual fire support from team members makes your armor situation more-than-adequate.
But if you play mainly solo, there are some SERIOUS moments of nail-biting in the early missions that are simply easier to manage with the increased durability of a Titan, or the (at that point in the game) superior melee abilities of a Hunter.
As a high level Sunsinger, though, I agree. Warlocks can be pretty balanced.
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I see where you are getting at however I think the misconception is the linear approach for warlocks leveling. As different choices come up tactics change. I think the difficulty for the warlock class leveling up is from the lack of versatility that the class is inherently amazing at.