Do I really need relic weapon for the maint story line!?

[Firepower]

The main issue for new 50's is getting up to item level to get into dungeons to get tomes to get better gear.

You do not need the relic for the storyline but getting up to i60 (leviathan) i65 (ramuh) i70 (syrcus tower) i80 (shiva) is a little more difficult now with the i90 gear being stuck in the new dungeons you cant access until i80.

With an i90 zenith relic (complete chimera, hydra, ifrit, garuda, titan, amdapor keep and a handful of soldiery) You will be well on your way to getting to i80 to finish the complete main storyline up to 2.4.

I would recommend getting the relic and upgrading it to zenith as doing the trials will give you an idea of the skill (dodging etc..) you are going to require to progress further past the primals in the story.