I was going to say - the chief one I see people siting is the ability to stay lighter.
I would agree with that, but I don't think it's as important if you are doint a full wort boil.
But the question is about partial boils...
I think the main 'con' is that it seems weird to many who have learned to boil all of the extract - like its cheating.
After doing all-grain for a while, I did my first partial boil extract for a demonstration. I did an all-grain pale for the demo, but had an extract version of the same fermenting (started a few days earlier). I was amazed how much darker the extract version came out.