Hell spear damage is based on your skill attack stat.
It does the following:
1. Does not stack with specials,(1MP and so forth)
2. Does not bypass monster or players defense
3. Will hit randomly by chance if you are grinding(Normal attacks)
4.(Addition to number 3) May also initiate by blocking or getting hit
Damage formula is based on
ATK STAT+SKILL ATK+ HELL SPEAR DAMAGE - MONSTERS DEFENSE
(Well i hope this right, anyway you get the idea)
My advice would be not to stack hell spear
Like only 2 of your items should have it or 1 would be better
Because:
1. It Does not actually increase your chance of hitting spears
2. There are more important primary stats
3. In pvp(if hell spear does actually initiate) it will only throw off
your enemies attacks(it will not however make them flinch)
The only good thing i can think of hell spear is:
1. It's uncounterable
2. If your character has a fast combo, then you'll get more chances(This is good)
Well it's up to you to decide, hope you were guided by this info.