Well, since you're asking me, I will always say getting every spell is worth it. My whole post is tracking my journey of getting every spell exactly when I could, or as close to it as I possibly could.
Blastbomb will give you another optional spell for MBing when you hit level 40. At level 40, Blastbomb and/or Bomb Toss are your spells of choice to MB your self-SC Fusion (Red Lotus Blade > Jet Stream > Fusion > Blastbomb or Bomb Toss).
Feather Storm, definitely. Feather Storm does good dmg, plus, gives the added poison effect. It can also serve as a spell for pulling if you find yourself in that position. The animation on it isn't long, so, you'll have more time to run back to the party without damage (range on it is nice as well).
As far as subbing Nin, /sigh, that's always a personal choice. Some will agree with you, and some won't. I personally subbed Nin from 28-75. I never had any issues with damage or anything. However, I do/did have Thf leveled to 37, so, if a party ever really did have a problem with /Nin, I could change without any hassle, but, I was never asked to do so. Whenever a party asked if I could sub /Thf, I always said, "Would you mind giving me the first 10 fights? If you still want me to change subs after that, that'll be just fine with me." To this day, after those 10 fights, I've never been asked to change, so, I guess that just depends on the player.
I'm ending though, saying, don't sleep on any sub. Give it a try first if you have any interest in it. My whole FF career though, I've always loved Nin and/or /Nin. However, I did take out the time to level any/all subs I felt, anyone would ask of me to have, so that I can be a very diverse player. Keep that in mind during your adventure. By the way, Blu owns! :P
Phoenix Server Rank 10 San d'Oria, 10 Windurst
75Rdm, 75Nin, 75Drg, 75Blu, 75Mnk, 75Smn, 75Sam, 75Pup, 75Dnc, 71Blm, 37Pld, 37Sch, 42Whm, 37War, 37Thf, 37Drk, 37Rng, 62NPC
Rise of the Zilart = 5/17/07 Completed
Chains of Promathia = 2/7/06 Completed
TOAU = 9/15/07 Completed