Maybe I can help answer this one with some hearsay and rumors that appear to
be true. I've played with charm quite a bit. Not the Enchanter charm, but
the druid version that only works against animals. My other char is a level
26 druid and I used charm quite a bit from level 14-18 (before I found
Lavastorm) Anyways...those are my credentials and here's my findings:

The Charisma modifier only affects the initial charm. *cast charm* [EQ
checks charisma] *charm holds* Charm has about 4 settings...ultra short
(less than 5 seconds)...short (20 seconds)...medium (4-5 minutes)...looooong
(10-15 minutes). The level of the mob in relation to the caster seems to be
a factor (green mobs are easier to charm and hold longer).

So, you ask, what the hell good is a mob that is 4 levels below what I'm
trying to fight? Gather round friends because here is the answer:

Buffs/debuffs

If you look at the druid spell list...level 19 (one rank above charm
animals), they receive a spell called Feral Spirit. This is a pet buff that
is supposed to help make your pet fight a bit above what it could normally.
This isn't to give you an advantage in an 'even' fight...this is to make
that 'even' fight possible. Enchanters are blessed with buffs that do even
more than this to help out their weak charmed pet.

I can see all of the raised hands ready to ask, "Why in the heck would I
charm a monster to fight when I can summon my pet?" Okay...this is part of
the hearsay that I was talking about. Your summoned pet will take a piece
of the action for itself. A charmed pet doesn't (as far as I know). You
can use a charmie to help you get 2 or 3 other kills and then when your
charmie is low and ready to die...release it (/pet as you were) and kill it
for full xp! A charmed pet will also respond to /pet commands.

Anyways...I hope this helps answer a few question about charm that might be
out there. I'm totally open to any feedback on this...this is just what
I've heard here and there and what I've been able to figure out myself using
charm animals as a druid...