Adventuring NPCs in the party are pretty much standard operating procedure around here, if for no other reason than if a party realizes it has a gap in its lineup (e.g. no Thief, or no Wizard, or could use another front-liner) they'll often go out and recruit someone to fill said gap.
What this means is that if for some reason everyone wants to play a Cleric the party's not then stuck with being all-Cleric: the flexibility is there to augment the party. That said, the players/PCs know there's drawbacks to overdoing this, mostly in terms of having to divide treasure and xp over more people.