Use case for Plough Driver, Groom, & Manservant in Agricola
I think you are being a bit harsh on these cards. Let's say you renovate to stone in round 11 - definitely doable. Ignoring any combos, for the cost of one action and 1 or 2 food, you could get:


Plow driver -> 3 fields for 3 food. That is likely to get you at least 6 net points (with extra if you sow at some point). It's also similar payback to one of the plow minor improvements, with the bonus that the action space required (occupation) is one that is much less contested at that stage of the game than the plow field spots.
Groom -> 3 stables for 3 wood. Similar to using the rooms/stables action to build stables (a reasonably common move in the last couple of rounds, in my experience), but a little cheaper.
Manservant -> 9 food (so 7/8 net). Not stellar, but certainly better than you'd get from something like fishing.


I think any of these could form a reasonable move, depending on the state of the game and what other actions are available. They certainly don't look horrible. Then, there are a few situations that would boost them from average to good moves. As well as the two cards you mention, there are others that might help you renovate earlier, and a few that help with playing occupations such as the tutor, bookshelf and writing desk.

I wouldn't plan my whole game around using them, but would definitely give them a go under certain circumstances, and particularly if the stone spaces crop up early, making prompt renovation more practical.