When I decided to bring a pure phase fleet for raiding, I thought about the choice between more cargo or more personnel (in addition to Efficiency Overhaul). Since the priority of the time was blueprints and not cargo (because I already had millions to buy whatever I need), I put the crew hullmod on all of my ships that did not need the campaign mod for Augmented Engines, for more marine capacity.
But if I wanted to design a new phase ship with expanded capacity, I would pick cargo because it can be used either to smuggle or take everything (not just the blueprint or special items) from a raid. Personnel would be only good for raiding. Phase ships have significantly less capacity than conventional warships or carriers, let alone haulers. A phase hauler with less capacity than others (but more than other phase ships) in exchange for no profile and better evasion seems like a good idea from a worldbuilding view. As for killing it, if it is not an effective combat ship that always runs away (like civilians do), it can meet Mr. Autoresolve, which I almost always use in a pursuit.
Imagine some one owning a very specialized vehicle like that, it would be so rarely needed that the economics for owning it would be terrible, so the company making it wouldn't be able to get any customers, and so they wouldn't be able to justify the cost of designing it.
I think phase field alone would be military only, but the military needs to haul stuff too. Not to mention outlaws who care less about legality and more about what they want. Yes, the phase hauler is very specialized. In this case, hauling without adding profile to a pure phase fleet, which is very specialized indeed.