I have no idea why an Oath of Vengeance Paladin would ever burn an action to cast this.

Quote from Cezmi >>

I have no idea why an Oath of Vengeance Paladin would ever burn an action to cast this.

 Not everything is about how a given spell interacts with your own class features. Sometimes it's about allowing allies to more consistently counterspell, banish, polymorph etc various important targets. Few things have high cha saves.

does this only affect the first attack/save the creature has to roll, or all of them for the duration?

Quote from Druidduo >>

does this only affect the first attack/save the creature has to roll, or all of them for the duration?

All of them, for the duration.

In the SPECIAL column in the Cleric spell list it does not show "M".

Then when I go into the spell itself it shows "M".

This is true of a number of spells in the special column in spell lists.

Is the special column in the spell list of any use if I have to go into each spell to find out the requirements?

Except spell to give to a homebrew Medusa. 

A little late, but spells with the M symbol in the spell list have a material component with a gold cost, which cannot be substituted by a spellcasting focus. This spell does have a material component, however it can be substituted by a spellcasting focus since it does not have a listed gold value.

Thank you.

In my oppinion, this spell is way better than bless. Bane is actually a buff, whereas bless is actually a debuff.

Bless = decrease enemy AC by 1-4, and decrease enemy spell save DC by 1-4.

Bane = increase your own AC by 1-4, and increase your own spell save DC by 1-4.

Con: the saving throw obviously. And the fact that bless debuffs all enemy AC and spellsaveDC, whereas bane only buffs your AC and spellsaveDC against 3 enemies. Still, I would take bane over bless every time. EVERY TIME!

Given a fair bit of monsters can get some scary to hits its more competitive with bless depending on party compositions alongside the enemies. Plus the whole entire preventing damage is the best healer approach is pretty valid.