I'd say "Allow, but infrequently."
In our Sunday (PF) game, we only have 3 players right now. So in the current campaign 1 guy is running an NPC who's been with the group almost from the start. Another just recently took the Leadership feat & so now has a 9th lv cleric cohort. The 3rd guy just isn't comfortable playing more than 1 character at these lvs at a time.
Sometimes, dependent upon RP/plot/etc there's an NPC under my control with the group.
When the group #d 4 players they tended to share control of the long term NPC.
Now when I say "running" I actually mean the combat/mechanical ends of things. The
RP of that long term NPC & cleric cohort? That's still done by me. But the groups pretty good about having them act in consistent ways during combat and suggesting appropriate responses, etc.
My Thur group is large enough that there's no need for anyone to play additional characters/NPCs. In fact they aren't keen on having NPCs tagging along at all.
Though if we end up running a 5e version of Skull & Shackles (a PF pirates AP) I think I will assign each of them an extra pair of pirates to control. That way (I) don't get stuck rolling stuff for 30ish crew like I did when I ran this for the actual PF group a few years ago. And it'll add RP activity when one PC is dealing with NPCs. Instead of me doing all the talking....
