Priest skill question (Doom and Summon Dragon)

I have seen multiple guides about Bishops and all of the skills for the previous jobs, and they all seems to be the exact same.

The question i have is why should you only take 15 in both Doom and Summon dragon ?

Why not just max one of them ?

Doom at level 15 will change the monsters to snails for 12 seconds, with 75% success rate.

Summon dragon at level 15 Summon a dragon will have a basic attack of 120 dmg for 80 seconds.

Do we take 15 because beyond that the difference is so small that it would be pointless, do we split the last points into both of them because of more options ?
Or are they both good but not as important/good as the others that we do max ?

I know that Doom could be good to use in areas with dangerous enemies, Basically 6 free-ish kills.

The Dragon however i do not know. I know that it is just a dragon that hovers over you and shoot some nearby enemies, but i do not know how usefull it is.

 

15 summon dragon is a prequisite for Bahamut.

https://bbb.hidden-street.net/character/skill/bahamut

 

+ u'll use 4th job sp to max out doom for maximising doom utility

 

Some more detail here. Doom is useful for removing elemental resistances from mobs, preventing them from using their skills (seal, magic attacks, etc.) and significantly reducing their physical damage. Basically a very useful mob control skill. Also, there was a recent-ish skill change which removed the magic rock requirement from the skill.

 

Oh ? So it bo longer consumes a MR ? That sounds great.