It's questionable if mobs should even get updated players spells. I don't think it's intended, but it happens all the time, probably due to the spaghetti-nature of 15yr old DDO code. It's also been "fixed" a bunch of times when it gets annoying for players, so I would not be surprised if that will be the case here as well. It's especially problematic on spells that have no save. Players are kind of spoiled by nearly always saving or fully evading most spell damage.
That said, if Melf stacks on mobs, its actually not terrible. Might even be slightly good. Without stacking it would quickly lose usefulness however, so I would think twice before removing it. One could maybe discuss just removing the stacking on players, or simply reverting it to the old version.
Acid badly needed a buff though, and it still has a very small list of useful spells, but mostly at higher levels. There was supposed to be a SL5 Cone of Acid in DnD (similar to Cone of Cold) but DDO never implemented that, and this left a huge AoE gap between Acid Rain at SL4 and Acid Well at SL9. Single-target is now pretty decent I think if both Melf and BDB stack.