Unbox as l-value in C#

What was rationale of having unboxed values in C# as r-values?

I've found for myself only two possible answer:

predict and prevent long term managed pointers to the heap
some primitive mechanism of unchangable value

But as for first - managed pointer to the heap appear in many other forms in the code (for instance as this pointer of structure that is a part of object in the heap), and as for second - I can change boxed value using reflection mechanisms.

Rationale

Ref types in heap are always considered as l-values. And in my opinion it would be reasonable to consider value types in heap as l-values as well. I don't mean that this is so ubiquitous operation (unboxing to l-value), but anyway in that cases where unboxing is used it could be valuable, for instance in collections (in their present implementation) of value types.

Implementation

Not big number of changes are required:

FUNCBREC::bindExplicitConversion - the part about unboxing (mark result as lvalue)
ILGENREC::genAddress - generates address for unboxing cast expression
ILGENREC::genMemoryAddress - generates address for unboxing cast expression
ILGENREC::genFromAddress - generates store instruction for reference of cast expression

That's all.

Example

Now I can compile and run following example:

public struct Rect
{

public int left;
public int top;
public int width;
public int height;

public override string ToString()
{

return "left: " + left + ", top: " + top + ", width: " + width + ", height: " + height;

}

public void Inflate(int x, int y)
{

left -= x;
top -= y;
width += x * 2;
height += y * 2;

}

}

public class unbox
{

public static void Main(string[] args)
{

Rect r = new Rect();
object o = r;

Console.WriteLine(o);

r.Inflate(1, 1);
(Rect)o = r;
Console.WriteLine(o);

((Rect)o).left ++;
r = (Rect)o;
Console.WriteLine(r);

((Rect)o).Inflate(-2, -2);
Console.WriteLine(o);

}

Questions and doubts

Looking from the more general perspective I do not see any need in box and unbox il instructions, cast instructions could handle this work well. What their rationale? Share your thoughts with as about this problem.