It seems that "Influence Ideas" are mainly for blobbing...

[Meow of Meowowiz]

Even if you don't play with vassals at all and don't need the actual diplomatic bonuses, the -50% to unjustified demands are very good and will save you a LOT of dip points in peace deals. In my opinion Influence is a great general-purpose group, though you don't technically need it if you just pick Religious and make heavy use of the holy war CB (ofc this depends on the country you play) and Administrative and then just core almost everything directly.

It's absolutely viable to play without Influence, but where I disagree is that it's not useful on its own and that it is a group designed for the HRE. In my opinion, Diplomatic is better for regular HRE members because you can actually make use of the additional diplomat and it helps you become the emperor. Diplomatic is also good for the PU game as others have already said which means that it especially shines in Europe (even if you are not in the HRE).

If you choose dip or influence is just a matter of what achievement you hunt for. If you don't hunt for achievements or allready got that few specfic ones than diplo is kinda crap compared to influence. Or if you really enjoy the gamble go for for dip but I can assure you that you're way faster with influence.(expecially if the event fires)

You just have to remember this only counts for central europe/hre plays. Otherwise both ideas are garbage and you can spent your points on alot more useful on adm/mil ideas.
It's kinda sad but dip ideas are just meaningless edit "in the end")except for exploration.