When using WeakReferences, one should be careful to not rely on IsAlive = true. If this property returns false, then the object has been GC'ed for sure. If it returns true, however, it could either be alive with a strong reference or marked for garbage collection.
There are several places in XF source code where IsAlive is used when it is true. In my opinion, these should be fixed and you should probably create some kind of VS macro to guard against future mistakes.
That said, there might be true use cases when IsAlive is true and you want to run some kind of code before an object is GC'ed. However, this seems to be more of an edge case than just checking the Target property.