Yea, for sure it's not suitable for all sites, but I think most could use it. I wouldn't call it very niche if Facebook is going to add it.
But of course it could be optional feature, controlled by on/off toggle at the settings.
I also wouldn't say that it could not be useful in a dating site for example.. think about a scenario where someone posts on their wall about date gone bad. You wouldn't "like" it, but you could sympathize it.
Then again, for example in gaming community, people post how they have tried something over and over again, always loosing and are tired of it... again, you wouldn't "like" it but you could sympathize it.
I actually truly think it would be suitable feature for most of the communities, but would be best when implemented as an optional feature with a on/off setting in backend.
But if you don't see it that way, it's fine with me.
Ps. could something like this be developed as an addon? e.g. have a Sympathize button in the stream items?