There are some trophies that are pretty hard to get. I'm talking about things where you have to try like 100 times before you can get it, and others you get just from one in a million luck. It would be really devastating to have those trophies and then some asshole hacker come along and delete them from your account.
OTOH, if hackers start giving themselves these really hard to get trophies then that cheapens their value and is a bad thing for those who actually earned them the hard way. So either way, whether a hacker is using it to harm someone else or only to "help" themselves, its still doing harm. If anyone can give themselves the trophies without earning them then there is no longer any point to them and just ruins the whole thing.