The only good reason is that the upgraded version has features you want to use - only you can make that decision
As to iPhoto '11 horor stories, they are just that iPhoto '11 is just fine and very stable - every new software release for every computer (like iPhoto was over two years ago) brings a rash of "horror stories"
If you are happy then why upgrade? If there are new features that you want then you should upgrade
LN
PS that being said I also believe in keeping my system up to date and personally think it is the best plan - but that too is a personal decision