Are the royal architypes worth it or necessary? //Game play spoilers//

[Raith]

The grind to unlock the royal archetypes on many characters seems very arduous. Are they particularly necessary?

[Houseki Yarisuke]

They aren't necessary. They can be very powerful though, as they have attacks that used to be synthesis abilities as a regular move instead.

[Outpost Omega 5]

Just a general strategy suggestion, don't try to grind out maximum level for all archetypes. Instead, stick with a primary archetype lineage for each character, with dips into other archetypes as needed or desired.

Any time a character equipped with a max level archetype of any tier acquires 1000 archetype exp, they get an item that grants a 1000 thousand exp to another archetype of your choice. In the second half of the game, over 90% of my archetype leveling was done through these earned items with out ever actually equipping the archetype. This was so I could learn a skill for my archetype of choice to inherit.

This is just my personal opinion, feel free to play as you want, but I focused on developing each character's "native" archetype lineage as their go-to option during my first play though. Now that I have the experience of a full play through under my belt, I would have recast Juna as a mage and Eupha as a healer over their "normal" archetypes. The last character you recruit who comes with the berserker archetype (I forget his name), I never really used due to their late game acquisition. When every other character in my party had multiple trained archetypes with a good balance of inherited skills, and I have this one new tag along with nothing, why would I bother wasting effort on grinding him up from nothing at this point in the game?

There is a bonus gotten from More for reaching level 20 in everybody's royal archetype.
The protagonist's royal archetype gets a synthesis skill requiring the entire team to be equipped with royal archetypes that deals a guaranteed 9999 damage. I don't remember what the SP cost was precisely, but at level 75, I remember I was able to get at least two, maybe three, castings of it before exhausting my SP supply.