It is 100% equal to both players to receive the Soul creatures and they give to both. There is absolutely NO benefit to any particular player. The only benefit is to someone who chooses to make a deck to use it.
um...there is a huge benefit to a particular player...that player being the guy with the light deck... most decks don't use light quanta (the ones that do are generally either hope decks, rainbow decks or life decks with rustler. The rainbow decks tend not to use light that much anyway, and RoL/hope decks and rainbow decks aren't pvp decks anyway, so chances are, if you pvp with this card, the opponent will find souls pretty much useless.) Most decks will not have much use of a weak creature, except for cremation or oty food. If you have a hope shield, their souls will do no damage and yours will power your shield and fuel your quantum costs.
You asked for suggestions, and that's a fair request, so here's my suggestion: you could make it so that, rather than being a spell, this is a permanent that makes creatures turn into souls instead of dying. Souls that die would, of course, die completely, not generate more souls (like the way that skeletons don't trigger graveyards). Rather than being mass CC, this would offer some protection against CC, but not complete protection. It would also be useful in preventing effects that are triggered by death (graveyards, condors. If you think that's too unfair on death decks, feel free to leave that part out). I feel that this would also fit the theme of light more too: rather than killing everything, it 'heals/helps' the dying souls. Obviously, this would work well in a RoL deck, but it could also work well in a mono/duo/trio light deck, a rainbow with empathic bonds, a mono-light, a life deck that uses light for rustlers, etc. When a creature dies, it allows you to regain light quanta to replace the creature.
hope you appreciate the input and sorry if i sounded a bit aggressive in the first half of the post.