Ninja, Dragoon, and Machinist all have short oGCDs that can make use of Spear in the case you want to cast it on someone. All of them have 20-30s oGCD DPS abilities that can benefit from having a shortened cast and if you clip any other buffs in the process, all the better!

Also, I'll just re-quote what I posted in a different thread since the 3.07 changes hit the table:

Quote Originally Posted by Ghishlain View Post
I always enjoyed using Spear back pre-3.07 to give my DRGs, NINs, and MCNs a nice small buff to their DPS, or get me a bit more MP faster.

Now that Balance, Arrow and Bole are both 30s buffs I'd rather just RR Spear and take a shot at the following:
  • 60s long Balance (effectively two Balances back to back)
  • 60s long Arrow (effectively two Arrows back to back)
  • 60s long Bole (indirectly reducing my MP consumption due to lesser healing requirement)
  • 300 Potency Refresh via Extended Ewer (25% weaker than 400 potency Luminous Aether)
  • 300 TP restored to a DPS (I believe Spire is 30 TP / tick)

As much as I love Spear, I don't think the effect Spear gives now compares to the massive list of buffs the above gives. Sadly, I'm pretty much going to regulate Spear to RR fodder outside of a few niche uses when the RNG just happens to land me with a Spear that I just didn't need at the time.
One minute DPS buffs / mitigation buffs are just too good to pass up now.