very good points, Nick.
the partial capo gives similiar effects to dadgad, but allows ease to play quickly in standard, without the problems of retuning extensively.
however with the partial capo you cannot use dadgad in first position with all open strings ringing, the first postion partial capo is effectively putting you in to dadgad in second position EBEABE, plus the ringing strings in this position EBEABE WILL BE LOUDER THAN THE PARTIAL CAPO STRINGS. only minor disadavantages but still disadvantages compared to dadgad.
   however i like the partial capo and find it useful, for songs like bold reynard the fox and lovely joan, that have a particular type of chord progression or in a particular mode[ it sounds like dorian[ but i have not botherd to check assidously,]
lovely joan, sounds dorian to me i think if i was in a dorian and using standard for lovely joan i would use approximately aminor and g chords occasional c major, but with partial capo a COMPLETELY different effect is obtained, HOWEVER TO SUIT MY VOICE, USING A PARTIAL CAPO I WOULD HAVE TO PLACE I Tbehind the ordinary capo ON FRET 7, about as high as it can go effectively ,exposing another limitation of the partial capo imo practically you can only use it on fret 2345.possibly 6 okay maybe for song accompaniment, but restricting you to five keys