I think it is personal choice. [BTW I use the program that came with PI called Photo Explorer to organise my photos and elements.]
I have never needed another program since I learned this one in a flash. I did the PIRC Beginner's Workshop, followed Jeri's scrapping tutorials and within a few months I knew so much about this very easy to use program. It does great things when I need my photos improved, and makes digital scrapping so easy yet it has such powerful features.
I guess the best reason is the fact the program is easy to learn and use while still being very functional.
But again, it is personal choice
