In order for that to work we'll need some sort of column concatenation operation.  This is actually non-trivial in full table generality.

A similar question would be, why `Label` instead of just using the more general `ReLabel`?  Here there are two reasons.

1.  There are cases, as in T['foo'] + T['bar'] where the schema drops from something like `{name: type}` to something like just `type` because we can't reasonably produce a name.  This is similar to the DataFrame/Series distinction in pandas.  
2.  Label is a Column, and thus has all the syntax bits while ReLabel is just a TableExpr, and can't assume meaningful syntax.  At some point we need to improve our handling of operator overloading so that operator methods like `__add__` aren't dependent on type.