Nepal ko lato, I would apply EAD if I was you. Is your employer willing to pay for EAD cost which is $380? If your employer is willing to bear this cost, then why not apply? Free is good.

If your employer does not want to pay for EAD then you can apply online USCIS.GOV website. Your I-485 has to be in pending status.

One good thing about having EAD card is; after your I-140 is approved, you can leave your current employer after 180 calendar days of Received Date (RD) of your I-485 application and join another employer with similiar job title. Your previous employer can't cancel your green card process. Who knows how long it will take to get GC thesedays with 2.5 million applications filed at USCIS this year? Folks who filed GC applications before June 2007 got lucky. And like you said .. "EB2 case will be approved with in time span of 14-16 months..", I strongly advise you to file EAD. Usually EAD card comes within 90 days of filing.

More info pon AC21 can be found in this website- http://www.murthy.com/news/UDac21qa.html#1