Let's compare skill,
DH, RF: Bombardet, 414% dmg
WD, Firebats: CoB, 500% to 1000% (after 5sec)
Both are channel spell with similar resource cost comparatively. The main difference between that two, apart from the damage, Bombardent is "range" while CoB is "melee". You stay at range and can move the Bomberdent area while channeling RF, while CoB need you to stay 1 spot and gradually increase dmg. Those seem to be very fair.
So what's the problem?
It's the game mechanism. You just can't stay range in D3. In D2, minions and mercenaries are mobs magnet and tank for you. That's not the case in D3. The game AI just design to have mobs attack everything. That's why even Blizz buff RF, DH's best chance is still SP tanking. That's why RF look bad compare to CoB. RF isn't practical while CoB's melee aspect fit the game mechanism.
Many people think DH eDSP suck (including me). But I have to say, if DH can do real "range" damage, they should have worse eDSP comparing to whose do melee, a fair tradeoff afterall. But as now if ranged classes(Wiz, WD, DH) are forced to act melee, they deserve to have better eDSP than melee classes(Barb/Monk) to compensate the 30% damage reduce difference.