Before I say what I'm about to say, this is not meant to be a war or flame thread that's meant to say, "this job is better than my job." I know this is probably among the least important things on a WHM's mind, but the light element nukes really need an overhaul whether or not it's in terms of actual damage or what their debuff does.
In terms of actual damage, every version of Banish and the spell Holy do a pitiful amount of damage regardless of whether Afflatus Solace or Misery is going. On top of the fact that the recasts for the spells are generally 15-60~ seconds per cast for so little. It's so bad that a BLM spamming Stone I (One, not I) can probably do more damage for less MP. Even against undead these spells do amazingly low damage.
The Banish spell line was originally looked at as a way to debuff undead so that the DDs can do more damage through the special defense layer. Truth be told, the debuff that the Banish spells do are almost entirely ignored due to the fact that people view these spells as nothing more than as weak nukes. There is no form of notification that the debuff from Banish has landed or worn off for people to understand that these spells are truthfully, a Dia-like spell (that stacks with Dia, without the DoT) against undead.
So you can see merit to using Banish spells, but not to using Holy. So what does Holy bring to the table in its current form? As is, it is a very fast casting light damage (in terms of actual damage and its element being light.) Its damage is significantly augmented by a charged up Afflatus Solace, but never to the point where the spell is effective. Usable by both WHM and PLD, but neither party will use the spell as a form of damage due to its low damage and its high MP cost. It doesn't do additional damage like Banish spells to undead. Used as a yellow !! trigger in Abyssea.
It seems that the spell was designed purely as a form of damage, but the numbers it produces make it so that it's not usable by players. The only time the spell Holy does a sizable chunk of damage is when Proto-Ultima casts it, or when a colibri mimicries it back at you with its damage seemingly multiplied.
A lot of players are looking at Holy in FFXI and comparing it to Holy in FFIV or FFVII and thinking why isn't it on par with Meteor. I'm not asking for that kind of extreme change, but the spell really has fallen quite a ways with its history in previous FF games.
So unfortunately the spell Holy doesn't do much to benefit anyone aside for being used as a way to get a quick spell off for skill ups. The fact that the spell was given to PLD means that the job is meant to be able to do something with the spell as well, but unfortunately I can not see it in its current form.
Currently the spell that does the most light based damage belongs to SCH as Luminohelix. I don't know if it's intentional or not to give the job a very powerful light based nuke because it's a hybrid class that's a jack of all trades. As is, SCH is the only job that can do significant damage using light based spells because of this. Not that light based damage is something that's highly sought after, but it's a fact.
To sum things up, the Banish spell line is an undead debuff that does little damage in general. Holy is a purely offensive spell, but is only useful in monster hands. I think anything at this point to makes these spells more useful would be welcome to the job.