It's just my opinion. While Sazh can use them rather well due to his innate high HP total (which can be offset with HP increasing accessories) I find it dangerous to rely on. If you do use them, at least one or two accessory slots are taken up simply to rebalance his health, which is a hinderance to be sure. However, when I was rather early in Chapter 11, I did use the Pleiades Hi-Powers (more specifically, the second tier version) to down a adamantoise for the first time. Equip these, pump him full of strength accessories. Then, in battle, summon his eidolon, dismiss it instantly, then raise the stagger total skyward. Change to Cerberus and let Sazh's unique Blitz hit the huge turtle many times over. You should just be able to down it. In that very specific situation, then sure, it's useful.

Perhaps in the short term, the sheer power of them can be useful to get through Chapter 11 and even 12, but they wouldn't ever be my long term option. When I was doing that hugeass guide, I was thinking long-term in mind. That's why Hope's Airwing can be useful during the earlier chapters because Hope has so little HP then that Critical: Shield can be rather useful. In the long-term though, Hawkeye or Vidofnir is far better.

At the end of the day, not many people like to upgrade multiple weapons. Assuming you only want to pump resources into one weapon and max it to the 3rd tier, then the Hi-Powers shouldn't be it.

Still, this is all my opinion though - I don't really claim to be an expert at all!