Can a PC have pet monsters?

As you know, monsters and PCs are very different.  Monsters have multi-attacks and night vision whereas PCs do not.  So it seems that allowing PCs to have “pet” monsters is a game-breaking setup as it allows players to navigate around ShadowDark limitations using their PCs pets.  And hiring a group of duergar or drow to guide the party through a dungeon crawl seems to me to be a step too far as well.  What are your opinions on PCs having/training monsters as pets or hiring monsters as henchmen?