I think a pragmatic reason exists for helping rebuild the Puerto Rican infrastructure, even at "public expense".
Consider:
What if a million or so inhabitants of the island decide to move to, say, Florida and Texas?
How do they vote, generally?
Even 200,000 more 'Ricans in Florida could easily "tip" that state into the democrat/communist column. That would surely hinder chances of Republicans winning future presidential elections.
Spend the $$$$.
Rebuild Puerto Rico.
Help the inhabitants there, to remain there!