When and how or if it should be excavated is a somewhat of a heated topic.

The Chinese goverment is doing the right thing (in most people's opinion) by waiting for technology to advance before digging into the hill.

In the seventies, when the terracotta army was discovered, the paint would flake right off as soon as the statues were discovered, due to exposure to air and the sun. Now they've got techniques where the paint is preserved as the excavation takes place. Similar advances in science and technology will most likely happen in the future. Not excavating the hill now ensures preservation for future generations. There's also already so much to dig around the tomb that it will take decades and decades to excavate what's there.

There will most likely be silk, fresco and paintings in the tomb, which are notoriously difficult to preserve once excavated.

The mercury levels in the tomb are also dangerously high, just how much mercury archeologists will encounter if and when an excavation takes place is anyones guess, but archeologists will have to come up with a solution to avoid the health risks exposure to mercury potentially pose.

The opposing view or side that support the opening of the mausoleum wants it excavated partially due to plain old human curiosity of what treasures and archeology is waiting inside, and not least the expected profit the tomb will bring to China, and especially the local area and the tourist industry.

Imagine if we had opened the tomb of Tuth Ank-Amon today, how much more information we'd be able to gather!

This article from livescience explains the case for not opening the tomb yet fairly well.