Although DHs can achieve the best sheet DPS as seen on Diabloprogress, the main problem is that farming is a combination of eDPS and spread (AoEing of that eDPS) and DHs are lacking in these areas; Blizzard "balanced" it by their increased mobility which makes them good at low MP. My friend Tangos is/was (selling stuff off right now) a ~700k DPS with Archery and Steady Aim DH. He could farm MP4 with ball lightning and one shot stuff; however, cranking up the MP to 9 or 10, things slow to a crawl compared to a Barb/Wiz/Monk at the 20B gear level.
Why is this so? DH's lack the resource generating abilities of Barbs and CM Wizards to cast the high DPS spells, they also don't have any good way to cast free/generator spells for consistent damage like monks or Archon Wizards and finally they have pretty meh damage steroids.
Looking at the first point, Barbs have into the fray and CM Wizards rely on ApoC so they can cast resource heavy spells constantly that have good AoE or little casting time. Barbs can deal high single target infinitely with smash for 406% weapon damage with 0 delay, or drop sprint tornadoes that deal ~ 20% * 9 * APS damage each over 3 seconds (for a 2 APS barb that's 360%). CM Wizards can constantly spam their explosive blast for 291% weapon damage combined with wicked wind for 252% and shocking aspect procs at 35% weapon damage each. If DH's had this kind of resource generation ability and they could spam cluster arrows and spike traps all day without relying on hatred generators then they would be on par. Unfortunately hatred can only be obtained through health globes (which are far and few between on high MP), passive on gear, base and companion and MoD which really isn't that great. ST-EB is pretty good for high mp in terms of cost efficiency, the problem is being able to find the choke point in the terrain so you can use it effectively.
On the second point, compared to monks and archon wizards DHs can't compete for free spells. Improved archon wizards shoot a beam at 375% weapon damage. Monks will use FoT-TC combined with SW-Cylone and Overawe to deal 350%+ depending on CrC (you'll have to refer to Meto5000s monk sheet for the exact numbers) just from autoattacks. DHs are stuck with at best 216% weapon damage from Bola - ID as their best free cast skill which has a delay associated with it. The GOOD thing is that blizzard decided to fix the stacking bola bug in 1.0.7 which makes this a lot better.
Finally Diabloprogress always displays unbuffed DPS. DHs items just have naturally put them up high there; however, when playing the game you're going to always have certain buffs and passives on. For a barb, that's 38% crit chance (with OP-KS and WotB), 25% ias, 45% damage (with Brawler and BR). From a unbuffed DPS of ~300k, I could reach close to 850k with those buffs. CM Wiz can have a combination of up to +74% damage (GC/conflag + BC + ST + CB (CB 20% is multiplicative)) or 20% crit chance (DF + PB), Archon wizards have +37% damage (GC + SF + FW) and 5% crit chance and Monks have +60% damage from overawe and exploding palm. Just a sidenote too, exploding palm is the most resource cost effective nuke out of any class. DH's have 50% crit damage and sometimes +12% damage from MoD since its not practical to cast it on every single mob. Most DH's don't go archery on high MP since kiting slows you down too much.
Besides coming a bit short in DPS output, the highest DPS skills for DHs have bad spread. For their best cost effective spender ST-EB, the range is 8 yards and higher DPS generator is 14 yards, which is quite lacking compared to other classes. Finally both of these skills have delays associated with them.
DHs are by no means bad farmers since they have arguably the best mobility of all the classes; however, as you increase your MP, DPS becomes and more important factor than mobility and DHs glaring lack of sustained DPS output becomes more and more evident. Since the game is balanced around farming, DHs can be considered on par with other classes; however, if your goal is to kill high MP monsters then DH isn't going to be the most gold efficient class for you.