You're assuming a lot of things by saying the Nexus will be useless. It WILL get more content patches to up it's iLvl and stats. We have 6 people in my raid team that actively do their relics when the new upgrades drop. Some way faster than others.
We downed T9 around 20 times and never saw a Summoner book. Not even one. Wanna know who our first person with Nexus was? Our Summoner. Not because the weapon wasn't dropping, but because we kept up with our relics. Doing your relic ensures you that you will have a viable weapon when content drops. Sure i115 isn't as good as i135 but the vast majority of people including the hardcore raiders won't have their i135 weapons for a long time and in the case of our group never. Having an option besides waiting for an RNG drop (instead of 6 Warrior Axes!!) is wonderful. I like knowing that I'm always going to have a weapon that will be good. Just because you can do the content, doesn't mean you WILL be rewarded for it.
EDIT: Just for reference I'll post our drops for you.
Scholar Book - 1st
Scholar Book - 2nd
White Mage Staff - 3rd
Paladin Sword - 4th
Bard Bow - 5th
Dragoon Spear - 6th
Warrior Axe - 7th
Warrior Axe - 8th
Warrior Axe - 9th
Warrior Axe - 10th
Black Mage Staff - 11th
Warrior Axe - 12th
Bard Bow - 13th
Scholar Book - 14th
Warrior Axe - 15th
White Mage Staff - 16th
and I don't remember how the order went after that but it was something we've already seen. The point is by the time we had our second warrior axe, it became pretty irrelevant what weapon dropped. Our SMN didn't have Nexus yet but at that point he really didn't care whether or not his book dropped. I was in the same boat. We had spent well over 2 months with the content on farm only to receive no benefit. There was plenty of gear we still never saw. We didn't need the new weapons, we could do the content fine without High Allagan. Nexus was a nice change. Something we could go for without having to rely on RNG.