Yes they have, this is the HERRO study.

They came to the conclusion that this would get as much work done as three missions to the surface.

To Explore Mars With Likes Of Occulus Rift & Virtuix Omni - From Mars Capture Orbit, Phobos Or Deimos

I don't know why NASA is so pushing the idea of humans on the surface. Because we are looking for life on Mars and human occupied spaceships can't possibly be sterilized to the same standards as robots for surface operations - especially if you consider the possibility of a crash. After a crash of a human occupied spaceship on Mars - I'd have thought that it would be the end of planetary protection of Mars.

We could find something unique there. Example, it might have early life that on Earth was made extinct by DNA based life. There is no sign on Earth of the lifeforms that must have evolved before DNA which is far too complex to be the first form of life on our planet. If that life still exists on Mars, then introducing Earth life could make it extinct before we can study it.

And more generally, we have often as humans made the mistake of introducing life to new continents, accidentally or on purpose, here on Earth. Why do the same on Mars before we have a chance to study it properly to see what the impact would be?

See my: Can We Risk Microbes From Human Crashes - On Mars? If Not, What Happens To Dreams To Colonize The Planet?