Comment by mike_hearn

It's not that weird. The goal is to enable existing types to be turned into value types without porting the users, so stdlibs and other libraries can mark types as value types without an API break.

That goal is an ideal and can't be reached perfectly. Converting a type to a value type will break clients that synchronize on them, or rely on identity for some reason. But such cases are rare, and can be weighed up on an individual basis when making the decision about whether to do it. Storing things in a nullable variable on the other hand is very common and changing the rules to prevent it would make every such change a source incompatible breaking change.