A main attraction of the Dweomerkeepr is its ability to convert spells into Supernatural ability certain number of times per day (max 4/day at 10th level).
This synergises particularly well with DMM(persistent) cleric because then 4 of your daily buffs are in no danger of being dispelled, ever. As arcane spellcasters do not enjoy as much cheese with persistent metmagic as clerics and other divine casters (+ sacred exorcist) do, it may be considered suboptimal to use primary arcane spellcasting in conjunction with the PrC.
However, the wording of the ability also enables you to ignore any components when casting a spell converted with the Supernatural Spell ability- meaning no costly material component *cough* forcecage *cough* or xp cost (as it's a component of spellcasting).
Yeah, and the capstone ability (Cloak of Mystery) is just the same ability as Incantatrix's.
In short, it's about as much cheese as Incantatrix and perhaps more! Except for its requirements perhaps