union of types in a different category

According to clause 3 on http://www.postgresql.org/docs/9.3/static/typeconv-union-case.html regarding union type matching:

3. If the non-unknown inputs are not all of the same type category, fail.

So a query "SELECT 1 UNION SELECT 1.1" works because 1 and 1.1 are of the same category, and one type has an implicit cast to the other, but the query "SELECT '1' UNION SELECT 2" fails because '1' is a string literal and 2 is a number and so they are different categories. Right?

Is this an artificial limitation of postgres or is there an underlying technical reason for this behaviour? For my purposes it would be better if the restriction was removed and that the union would work as long as there was an implicit cast that allowed conversion of all fields to the same type.

MSSQL doesn't have this restriction and I'd prefer if I didn't have to rewrite these queries (or create a complete set of mssql compatible types in the same category) when porting applications.

Thanks

James