I know you didn't say that, that isn't what I was arguing against either.
I wasn't intending to respond for Mileena, by the way, just offering my own thoughts on the matter.
Most of the boss souls weapons are
terrible, though. Hint: only Queelag's soul and Priscilla's soul are worth anything (and that's only for Dex builds). And, to a certain extent (for Intelligence-heavy builds), Manus's soul for a powerful spell, and the Moonlight Butterfly's soul, that spear isn't too horrible when fully upgraded, but even then the Moonlight Greatsword is a better weapon for int-builds. I guess Gwyndolin's soul is ok if you really, really want a bow for your faith build... then again a Divine +5/Occult+5 composite bow is probably better anyway, but you'd need an extra white slab.
Unlike in Demon's Souls, where most boss souls
are very useful, most of the boss souls in Dark Souls can be safely consumed for their souls (seriously, they're that bad, that was a little disappointing).