Tamamo (Caster)

I'm surprised you obtained her bond CE before maxing her.
I'm close to maxing my Archuria myself (10/6/10), but she's still sitting at Bond level 5. XD

Imo Tamamo's skills are worth it because they are so useful. 90% is already a pretty good chance to reduce the enemy's charge, but it's still RnG and therefore a risk you're taking in every battle. Plus, you get the reduced cooldown. The def-up of her second skill from lvl 6 to 10 is also solid.
I'm not a huge Tamamo fan but I'd do it if I were you. ;)

W(W)a(i)i(f)f(u)u >>>>>>>>> ANYTHING.

Choose your beloved to grail and 3-10.

Okay back to Tamamo. MAYBE her 1-2 skill are not critical as 3rd. But 1st skill is excellent for stall team, or normal Art NP spam team. 2nd skill also beneficial for herself, since supports are generally not having class advantage in boss fight, need better protection.
That is why in most cases for me, Art team's Waver always the first one to be defeated since no class advantage and no such 2nd skill as Tamamo. And rumor for future boss fight, enemy will focusing on damage your supports first, not DPS with class-disadvantage. So eventually you need to get her 3-10.

I think its definitely worth the cost. Most people only care about her third skill, but in truth, all her skills are great and since they cap out at five and she can spam her NP like crazy, you're usually looking at keeping her skills up most the time. Her defense keeps her longevity going and while reducing NP is going to become less important from this point forward (post camelot) its still saved me many times. Honestly, because of her NP, her skills especially shine when maxed out, definitely worth the effort.