To risky for amount of work and payoff....

Wild Mass Guessing:

CONCORD very carefully monitors what sort of equipment/ships/modules are available to Immortals in general, and categorize what each of them can do. This is why we don't see true custom ships flying around New Eden under capsuleer control, only a (Relative) handful of predefined hulls (With many more that aren't available to Capsuleers)...A more dust related example would be the weapons (Such as the Arc-Cannon) and vehicles (Amarr Tanks, Speeder Bikes) that haven't yet been sanctioned for use by Empyrean Mercenaries for one reason or another. Additionally, we know that CONCORD doesn't allow the purchase of Empyrean Mercenary tech by Capsuleers, nor does it allow the purchase of Capsuleer tech by Empyrean Mercs...so I guess one reason would be that CONCORD doesn't want capsuleers to have access to the Warbarge class of vessels because it doesn't fit in with their standard models...and Probably because of the mobile production and research capabilities...also, the primary purpose of the Warbarges fitting into certain CONCORD/DED enforced standards is so that CONCORD can keep track of Immortal Mercenary Activity...so they don't abuse their Immortality too much.

Btw...on a somewhat related note: I think the closest Capsuleer Vessel to a Warbarge is an Orca Class Industrial Command Ship, so I choose to believe that the Standard Mark of Warbarge is based on an old CONCORD Disaster Relief and Command Vessel Prototype that was never actually put into production, until it was modified for the purpose of keeping track of us Damn Immortal Footsloggers.

So...the question of why would a Capsuleer Want to pilot a Warbarge, stay on-station for 15+ minutes, relatively stationary, in space, in something approx the size of an orca...only to have to warp several jumps to go to another valid battle?

My first guess would be that it would suck for dust mercs if their battle ended before it started because the Warbarge got caught in a Gatecamp or got suicide ganked in HS...

Another would be how would you properly reward the Capsuleer in such a way that sitting a relatively expensive ship with very little defense (if we assume the bombardment array consists of Small guns...which based on the relative strength of what the Warbarge can provide compared to a Destroyer is making a big jump)...you would have to have sufficient reward to help mitigate the Risk associated...and to where there wouldn't be a more profitable activity that's safer/easier/less time consuming so people would actually want to risk an 800million+ investment (if our assumptions about the Warbarge's Scale are correct) to have gameplay that is about as interesting as mining.

Shame. Would have been really cool to get the doomsday working, and turn it into makeshift orbital artillery.

Well, let's look at it in relation to its most similar other activities: Mining and Hauling/Freighting

A High-sec freighter pilot can expect to make about 1 mil per jump, with rates skyrocketing up to about 35-100 mil per jump in Low-sec (however, for LS freight services, it is far more common to charge by the LY and use Jump Freighters, and I'm assuming Warbarges lack a Jump Drive), however, that's shared between the fleet that would be escorting a standard freighter (Webber, Logi, and/or EWAR vessels). Then for remaining in orbit, a Warbarge Pilot would need to get a minimum of 500K ISK per Minute in order to match the yield of Veldspar (Assuming Mining Veldspar in a Hulk Class Exhumer, which is a fairly reasonable comparison based on the estimated cost of the Warbarge)...and that's just for High-Security Battles...in Low-sec/Null the Warbarge will need to have almost a full Battle-group to defend it, making that rate have to extend to all pilots...so 500k ISK * n Number of Capsuleers in the Battlegroup + (1mil * y Number of High-Sec Jumps) + (35-100 Mil * z Number of Low-Sec Jumps) + Travel Compensation for the Battle-group and allowance for Expended Ammunition. All told, that's quite a bit of investment, for not a lot of return (compared to other available, safer activities)...I don't think it couldn't happen, I just think it would be impracticable to incentive Capsuleer pilots to do so

Its going to happen. Accept this time we need YOUR support dust bunnies...