Re: NorthPoint Cambridge (The one that was train yards, the big plan.)
I'm a preservationist, so I've really only observed it being done as well. The genesis of the issue is that when they first started working with reinforced concrete, and for quite a while afterwards, they put the rebar much closer to the surface than today. Additionally there were often more contaminants in the mix, particularly salt, as well as too large/small aggregate and other issues. Between that and concretes natural porosity, prewar concrete is far more susceptible to spalling.
If the job is done right, (not only just grinding down the spalled concrete and exposed rebar, but the adjacent areas as well, plus using a good quality binder) it should last at least as well as the original work. Patching/extending concrete is never easy nor perfect though, it doesn't like to stick to itself once cured. Of course like anything, maintenance is key. Keeping an eye on the patches and regularly sealing/painting them is just as important as the patch itself. I'm sure some of the other guys who spec this stuff will chime in with a more in depth process of how its done, for instance on larger patches I believe they use anchors and such.