There a couple of quests in Silverglen and Black Cove involving Star Stones that requires Pet Pal, but neither are mandatory. You can get the Star Stone regardless, you just don't the extra quest XP from talking to the animals. The cat in the Inn starts the Black Cove one and Alfie in Luculla Forest starts the other one.
As for the other posters' suggestion to avoid Charisma, it depends on what your end goal is. If you want to maximize XP gain, the best path is actually to successfully negotiate the charisma option to gain the full Charisma XP and then slaughter them all afterwards to get the combat XP.
Don't put Charisma or Pet Pal on henchies.
Barter IS nice, you should save all your +barter equipment and consider investing 1 point on your barter character (it's a small price). There are some end game vendors that sell very, very nice things for lots and lots of gold. I have over 100K on standby but a transaction can easily go over 60K for 2-3 nice pieces of equipment.
Do not sell back Tenebrium weapons to vendors who can't handle Tenebrium. They will get the rot and die, meaning you will no longer be able to trade with them.
You should know there is a way to do *huge* amounts of damage using Telekinesis. It's kind of an exploit right now, but the option is there if you want to go for it.
