Of course if you operate in more than one country (or even in more than one state in Australia - don't know if everywhere in the US has the same DST rules), this table would get way more complicated where you have different places with different DST rules.... You'd have to add in a time zone name column too, which would then also need to get stored along with your datetimeoffset value in every table where you want to use the information - back to needing two columns to store your time data.

Having a 'DST' bit in the datetimeoffset type would be the best resolution. I would have thought they could add a bit quite easily - perhaps an extra bit of 'precision'.