Give me a reason to use the new data type when I can't use date functions with it natively, or can't get DATEDIFF in particular to work with it at it's higher level of precision.
When a datetime is needed as the leading key in a table, such as for logging tables, datetime2(7) causes far fewer first-key-column value collisions than does datetime (yes, if you want to insure unique key values yourself, you still have to add $IDENTITY or some other unique value, but that's not likely needed with datetime2(7) vs just datetime).
SQL can often process a lot of rows in 3ms. Thus, I nearly always use datetime2 for such columns.
SQL DBA,SQL Server MVP(07, 08, 09) Prosecutor James Blackburn, in closing argument in the Fatal Vision murders trial: "If in the future, you should cry a tear, cry one for them [the murder victims]. If in the future, you should say a prayer, say one for them. And if in the future, you should light a candle, light one for them."