Aspergillum requires you to have atleast 1 point into Aspersion to work, more points into it does increase the DMG Aspergillum gives, however as a SPR Chaplain it would hinder your utility a lot because you’d need both Aspersion and Blessing lvl 15.
But it all depends on what you do the most, if you solo like most of the time as in doing merc missions or dungeon runs alone you’d benefit more from more DMG, but if you do party a lot or want to go for ET and be efficient there you leave it at lvl 1 and get higher lvl Mass Heal and Revive.
So in short, if you solo a lot more than you party up with people max it, if you party a lot with people and want to perform better as a support-ish kind of Chaplain, leave it at 1 and use the points elsewhere, keep in mind that maxing it is not completely necessary unless you really want more DMG and do not care about leaving other important skills at low lvls or untaken as long as you can see bigger number.