I use identities for the same reason that people are issued SSN's... It makes life real simple as a unique identifier in the world of large batch programming. As I prepare to install handrails for the upcoming ride :-P, I'll also tell you that if a natural key exists, I'll still usually use an IDENTITY column as at least a Non-null Unique Key and make the natural key the PK unless, maybe, if it's a totally static table.
I'll usually put the clustered index on an IDENTITY column especially for large tables that are also highly transactional just to keep page splits at the lowest level possible. Sure, a date column would probably suffice but there isn't always a date column and, when there is, data is not always inserted in the same order as those dates. To me, it's worth the 4 bytes per row to keep from having nearly 8k bytes suddenly open up because a single row insert caused a page split. The extra 4 byte column is usually worth it to me in the savings it causes during maintenance.
I also tend to use IDENTITY columns on everything I import. I always import to staging tables and never to the final table so that if someone does send "dupes", I can easily find them and tell the vendor of their problem. Yes, there are several ways to avoid the dupes but I always want to capture them for one reason or another and the IDENTITY column provides uniqueness when there may otherwise be none. The use of IDENTITY columns also makes it a snap to archive data because I can usually archive rows on one logical end of the table while the users continue to use the more recent other "end" of the table without mutual interference.
The bottom line is that as with everything else, "It Depends". I can see arguments for using IDENTITIES for all tables, no tables, or just some tables. I happen to be one of those that favors the use of IDENTITY columns.