Try an experiment. Level a mage. Then imagine leveling a mage without Frost Nova, Polymorph, Frost Bolt, Arcane Explosion, Cone of Cold, or anything resembling crowd control or good AoE. That's what leveling Destro is like.
I should be fair, warlocks have decent CC in Fear (and later Howl of Terror), it's just nowhere as safe as other classes CC. But we have nothing like the control capabilities of a mage's sheep and frost spells. Destruction plays much like a mage, but with none of the "Oh, cr*p!" abilities.
All that said, try destro out for yourself. I know of a guildie who leveled a deep destro build, and he loved it. "One man's trash is another man's treasure," don'tcha know...