Is there any use at all in having 1" of XPS over a foundation wall above grade from a dew point ....point of view?

Depends on what you're trying to keep above the dew point.

Concrete is highly tolerant of moisture, and dries readily to the exterior on the above grade portion.

But R5 on the interior side may be important for keeping stud edges of an interior finish-wall above the dew point and saturating over a cold winter without something to keep them a bit warmer.

What's the foundation wall stackup, and what's the geographical location/climate?

It is in Toronto. I have 2" of XPS below grade which has worked wonders for the basement heating but the foundation guys stopped at grade and switched to 1" from there to the rim joists and it has been flashed and sealed (I was away so it was done without me knowing it was only 1"). I will be putting 3.5" of PUR foam from the rim joists up to the flat roof but I'm not sure what the effect of the lesser insulation will be on the foundation. I don't have any insulation on the inside of the foundation.

Is the XPS on the interior or exterior side of the foundation wall?

I would add 3" more of rigid foam on the outside, from the ground level to the rim joist. For insulation reasons. You could bevel the edge and cover it with stucco. Any heated space with only 1" of rigid foam separating it from the outside isn't good.

You have to be careful about providing drying paths for ground moisture wicking into the foundation wall- a blanket recommendation for 3" of exterior foam requires qualifying the stackup (which I'm still trying to squeeze out of Mike), or knowing what (if any) capillary break materials there are between the footing & foundation wall, and between the foundation wall & foundation sill.

I agree, a drawing or some pictures would be good.

The XPS below grade is covered with what looks like the Bakor roof cement, tough stuff and sealed right down to the footings. There is dimple board on the outside of that and it was stopped at grade. The house is 100 years old so I suspect the poured foundation (pretty soft) was poured right on top of the footing. It does have new weepers but code in Toronto is for disconnected downspouts so all the rainwater is directed 30 ft away from the house. All insulation is on the exterior, none inside.

I definitely want to add more insulation than the 1". It isn't what I asked for but they are long gone so i will rent a brake and do any flashing myself.

If it's all on the exterior, have at it- put as much insulation out there as you like. It has nothing to do with dew point, but everything to do with energy use. The warmer you keep the concrete, the drier the foundation sill will stay.