5 more points is hardly anything to be concerned about. Maybe at level 10/11 when your Cle/FvS first gets it, but such a small increase will quickly become irrelevant in its impact to your SP pool. Every 7 heals, you effectively lose the SP to cast an 8th...ohnoz!
As for why increase it, heal just so totally outclasses all of the other spells, that it's a bit silly. Leaving aside the large amount of HP returned, the laundry list of status effects it cures frees up a ton of spell slots that might otherwise be spent on a half dozen other spells whose only purpose would be removing icons over player's heads.