It depends on how long of a stub you have coming out of the wall. Obviously it needs to be long enough to accept the shut off for the toilet.

We are remodeling our 1930s kitchen. The plaster is cracked and crumbly in some good-sized areas (2' by 4' and 3' x 5' approx). Our trusted electrician insists that the kitchen contractors should demo the plaster and replace it with drywall in the worst areas. The kitchen contractors (whom we've never used before) do not believe this is necessary, and that patching the plaster will be sufficiently strong. They say that if we apply drywall to the old plaster, the new cabinets will not fit, and it seems to me that the extra weight of drywall on already-cracked plaster will only worsen the problem. They caution against doing a complete plaster removal due to the expense and amount of dust. (2 people in our household have asthma, so this is an issue). One wall is exterior brick with no studs beneath the plaster. Another is terra cotta block, also without studs. Metal mesh lathe is used throughout. Because there are no studs, how can they anchor the new 42" solid cherry wall cabinets with either scenario? Without studs, will the brick-plaster or brick-terra cotta systems hold up? (Our previous cabinets were 32" metal). Chat room advice is as varied as my contractors'. I don't know whom to believe!

Unless it's on the ceiling, I would not worry about it. Patch or fill in with 45 minute joint compound, or plaster of Paris. When mixing - always use a clean container. Old joint compound mixed with the fresh causes the fresh mix to set up faster. Yes the the plaster should be stuck to the lathing, and in fact it's hard to imagine how it could not be, considering how it's applied.. The lathing is also there just to stop the plaster from filling up the entire void between the wall studs. Ceilings would be my only concern and you would notice it if your plaster or lath was sagging. And that's repairable too with large washers bolted into the lathing or screwing the lathing back into the joist.

Air should not be able to pass freely into the stud space. That could increase the spread of fire. If the cabinets are not enough to seal off the space, patch the holes.