Married to US Citizen, what happens if I don't apply for the Green Card?

[ca2014]

Hi, I entered the US on an R1-visa but choose to marry an US Citizen and that Visa is now terminated.
We are ready to apply for the green card for me but my lawyer said something to me that makes me wonder: "There is no rush, some people wait ten years before applying for the Green Card". So, why do I have to apply for the Green card apart from the obvious benefits of getting a work permit etc. Is it true that I wouldn't be able to leave the country and come back? Can I choose to not apply for the Green card and still be here legally?

[immihelp]

If your R-1 status has expired, you are already illegal in the US. You should apply for green card right away.

Of course, you need a different lawyer.