The real answer to thsi issue of should I go with the smallest data type possible (for space & badnwidth reasons) or to go with what is portable and or last greater then 100 years (in the case of the DATETIME data type) is that "It depends". For some portability is more important then space conservation.

That being said there are still a few cases where no matter your situation I think we can all agree that changing the data type to something smaller is a good idea. In our environment the system we use has every single Boolean filed in every table set to use the INT data type or is set to use Numeric(18,0). Talk about a pooor choice and a waste of space! You know how this happens? You let procedural progarmmers with no proper DB Training making design decissions about the database. Every one of these Boolean fields should have been set to a BIT data type or at least to the smallest integer value in the system like TINYINT.