The argument against is that it is written into the Constitution as such. The reason being that as the seat of the Federal government, DC has 435 Representatives and 100 Senators already. As to taxation without representation, they have been granted the right (through constitutional amendment) to vote in presidential elections. They also have a devolved government that they elect directly.
Puerto Rico is a harder case, but the people of the island have not been in a hurry. When last put to a referendum, 55% of eligible people voted and they voted only by 52% for statehood. It is only in the last 10 years that there has been a slim majority in favor of statehood at all. As for representation, They don’t pay federal income tax and they don’t vote in federal elections.
My 2 cents on the matter is you have to pick a political philosophy and then apply it consistently. If you support federalization of elections, then you should also support federalization of the federal district.