Datetime as Primary Key?

"In general, you should avoid using datetime as a primary key. First, datetime is an 8-byte data type, and narrow keys tend to be more efficient and faster than wider keys. If your table is going to be very large, a smaller integer-based data type, such as the 4-byte int or the 2-byte smallint, might be a better fit. Second, and much more important, datetime is accurate only to one three-hundredth of a second, or 3.33 milliseconds (ms). By definition, primary key columns must be unique, and you can't ensure that you'll have unique values in a datetime column. Your business rules might say that entering multiple records within 3.33ms of one another is impossible,
but I think that making that assumption is dangerous. Business rules and technical assumptions can always change."

From SQL Mag's 38 Extra Development Tips