I haven't tested on my 60 yet
Lvl 21 Necro, 77 Charisma, mob 6 levels lower: Charmed Pet did 35% more dmg than summoned on max hit, broke constantly, and wasn't worth maintaining as it took away from dps/cc/healing in the group. Overall dps was lower than 35% with many single digit hits.
We did have a lvl 20 Enchanter in the group with 89 Charisma: his pet broke more frequently and he gave up on it after 30 minutes to summon a regular pet.
Giving pet wpns actually reduced the dmg output and no duel wield/double attack was present. Most likely this is due to lvl of the mob (15/16) not being high enough for the skills.
TLDR: Not worth using at lvl 21. Forcing charmed pet into the only remaining avenue of meaningful exp gain (Charm Stacking) doesn't make sense and is counter-culture to the spell.
WIll update after testing on 60