Vince,
I think the point I'm getting at is cost. By building a trailered hoe, you eliminate the strain from the 3 point and/or the need for a subframe mount. By eliminating the engine on the trailer hoe and using the tractor's hydraulics, you retain the 3 point/subframe benefit, plus eliminate the cost of an engine and pump for the trailer. This would make implement changes a matter of only a few minutes. It would have the negative benefit of making the trailer hoe entirely dependent on the tractor, but no more so than a 3 point hoe.
SHF