"why would any senator give up 2% of their power to add 2 more senators?"
Presumably for reasons similar to ones that lead previous Congresses to add the previous 37 states to the Union, despite even higher dilution of power.
Manifest destiny? Vast natural resources? Strategic location? I don't see a compelling political reason to do it, but fortunately we're free to disagree.
If DC's Members of Congress are likely to vote with the Democrats (which I think they are) and Congress has just enough Democrats to allow DC to become a state, but a couple of offices are shaky and they are likely to loose power, then bringing in DC as a new state would help keep the Democrats in power.
That sounds like a decent reason. Not compelling, but decent. It only needs to be enough to counter the anti-dilution argument, after all.
Puerto Rico's statehood is more likely though. In the 2012 election both Obama and Romney indicated support for statehood should that be what the people of PR decide. That's complicated because apparently the Republican Party of PR is the statehood party, even though PR is likely to vote for the Democratic Party. (See http://www.caribbeanbusinesspr.com/news/dc-lobbyist-warning-... )
So, D's control the House and Senate, DC and PR become states, and the D's pick up about 10 seats and hold power even longer. That's more compelling than DC alone.