IIRC it was a change they introduced mid 2013, along with other preparatory changes for the eventual Unity 2D environment.

I'm not seeing it specifically in the change logs, but there is a definite cutoff point where in the unity3d forums people stopped complaining about the difficulty of converting the Vector2 mouse coordinates into vector3 objects.

Waaa? Is it really that hard to do mouse3 = new Vector3(mouse.x,0.0, mouse.y) or however you want to divvy up the values?

Also, I am in complete agreement with Hodgman, I really wouldn't have minded if they had a .ToVector2() or .ToVector3() function, but doing it behind my back is what drives me crazy.

This happens all of the time in shader code, and I firmly believe it to be Evil. It has been the source of many bugs that were difficult to track down. Fortunately the HLSL compiler will emit a warning when it truncates vector types, so if you enable treating warnings as errors then you can catch it.

Yeah, I'm a firm believer in catching as many errors as possible at compile time, which is why I like languages that are strongly typed.