They will probably not attempt any cloning for at least the next few years, even if someone wants to do it - cos the technology is still not up to scratch for that.
It will take them a couple of years at least to get DNA (if possible), check if what they got is correct / not contaminated / study and understand it.
After that only will the question of cloning become relevant.
As for cloning something which has been dead for millions of years - I will say go for it. But make sure you take proper safeguards so not to get eaten by an escaped giant lizard.
As for the benefits - it will tell us how accurate of theory of evolution is. It will help to classify / study other animals in a similar evolutionary tree. It can provide new insights as to muscle / bone structures and how they can be improved on (useful for creating artificial limbs). This is cos T-rex is very different from anything we have now - it's body structure has to be very interesting to be able to support it's mass and allow it to move fast (assuming the movies are right about how fast a T-rex can move).
And who knows, it might even be a good food source in the future.
There are so many possibilities - some which we have yet to forsee.