I might use an oil primer or BIN on the staircase and I might use an oil based paint for the dark brown color, but latex on the rest. I'm sure the painters reason for using an oil based paint is that he thinks it will be more durable. The guys out there that believe that truly believe it down to their soul. To me personally, I don't think that the yellowing is the worst thing about using an oil based paint. It also chips easily because the paint film is brittle, which means that caulked areas like mitres and gaps will crack sooner than if you used latex. Plus, this is the year 2010. Oil based paints are just about dead.

Also, that counter will be replaces with granite.

I talked to the painter today, and he said he would use latex if that's what we prefer.

From what i've seen, most commercial offices use oil base paint because of the longevity and durability properties. I'm thinking of deliveries that may be brought in and walls, etc., getting bumped. Just my 2 cents. ;o)

That would actually be one of the main reasons I would not use oil. People are constantly banging into the trim with the vacuum, items, etc. Nobody treats office space like it is their own home. Since oil based paint is brittle, it will chip easier.

Oil enamel (pigment + varnish) stands up to cleansers better than latex. Oil will not be degraded by ammonia or caustics like latex will. So in an environment with repeated handling and cleaning, oil's going to hold up longer. You may need to repaint every five years instead of ten (home environment standard). Maybe they have acrylics that are ammonia-proof these days. And maybe low-voc cleansers are more acrylic-friendly. But in commercial apps I always think about the handling/cleaning cycles.
Casey