In the ArM4 game I'm playing that is still going on, I understand that "constant" frequency enchantments are pretty much like that: unlimited frequency in a constant state of auto-triggering, in which I assume that the lower cost can be explained by the inability to control that.
I think that D:Mom + unlimited uses would make the most sense for effects that are intended to be used on people regularly in adventures but not really for long periods of time. For example you might want your equipment to adapt to your Bjornaer's transformations during a quest without worrying about daily uses, but for the rest of his life, you won't be really in need of that. Warping shouldn't be that much of an issue, in fact I recall reading somewhere that "effect modification" levels don't count when considering if the effect will cause warp, since those pertain to the device itself, which means that this method produces effects generally two magnitudes lower than the book's "constant" method. In the other hand, these should be for personal use only, since anybody else would get twilight points so fast that probably Criamon are already doing it, lol.
But for the "tales" thing, I wasn't talking about tales within the game paradigm but IRL. Magic in Ars Magica uses a very original and complex method, but it's still based on our fantastic concept of magic that is used in fantasy and myths. My point is that this is something that is very tied to the whole concept and I'd consider it strange to see it ruled out. And yet, the idea of a faerie enchantment being constant seems to me even weirder, considering how "inconsistent" faeries are described.
I don't know, personally I don't see any problems with doing as I described, taking advantage of the ward mechanics if necessary. Is it stated anywhere that wards can only repel things? Why not wards that turn arrows into flowers, melt swords, or turn people into pigs, instead? That's also something very common in magic lore.