EzekielRaiden
Follower of the Way
The downsides of "pet as extra PC" strike me as both simple and obvious, but perhaps I am thinking of it from a different perspective. (1) Bookkeeping; every player with a "companion NPC" (whether or not it's a "pet") requires about double of most everything that takes up time in a session, be it sheet checking, RPing, treasue/xp allocation, etc. (2) "Impact." If the companion is a full (N)PC in its own right, it *more* than doubles a player's ability to accomplish tasks and overcome obstacles, because companion character (hereafter "CC") and PC can work together, e.g. flanking enemies, Aid Another, more chances to crit, etc., so there seem to be no reasons NOT to elect to play a CC along with your PC. That is, the game gives incentive to play one, as the only "player cost" is the oh-so-onerous "I have to remember that I'm twice as powerful because I'm really two 'people.' " (And yes, I understand that that IS onerous for some people, but I find it unlikely that it would be more trouble than benefit for most players; it would be like playing two separate hands of blackjack at the same table, not for everybody, but significantly advantageous to those who do).
Another way of saying this is: if players get all the power of a second PC with it, what are you going to do when everyone at the table now wants a companion character and you now have 8-12 "PC equivalents" in every combat, skill use, and roleplay event?
Another way of saying this is: if players get all the power of a second PC with it, what are you going to do when everyone at the table now wants a companion character and you now have 8-12 "PC equivalents" in every combat, skill use, and roleplay event?