The main issue with Recovery Conversion, imo is the random element to it. If every mob type has a fixed Recovery Conversion type (e.g. Vengeful Soul is always vulnerable to melee attacks but immune to ranged and magic attacks) then I think the skill is defensible as Pit would still demand a balanced party of different sources of damage, ergo mitigating the original issue of Pit being a party with one Sura to tank, one AB for support and ten Soul Reapers for DPS.
The random element to Recovery Conversion, at least in my experience, is too much for a chaotic 12-man party to handle. You see a mob, everyone wacks it, guessing which damage type it takes damage from, some learning faster than others, while others are still healing it. With all the eye-blinding effects present in the game, plus the chaos of eleven other players and the occasional happenstance of a rogue monster killing half the party from behind, imo it's a bit much for the mind to process quickly enough to adapt to in a timely manner. I'm also reticent about visual indicators as a band-aid solution simply because not everyone can handle playing with effects on.
If you know the mob's vulnerabilities and immunities beforehand then you can plan around it, and if they're healed because you use the wrong damage type to deal damage then I think that's an acceptable penalty for end-game content that seeks to diversify classes and builds necessary to clear said content, as it would then be the players' own short comings for not planning around it when that information is available. A good chunk of Ragnarok Online, as with any MMO, is learning about the content you're trying to tackle beforehand and planning around it. That's my two pence.
