I would like to have seen more comparison of the difference between included columns and additional columns in the index key.

I appreciate the distinction that they are stored as "extra" data and the benefit comes from not having to move rows in the index when the extra column is updated in the base table. In the case of rarely-write, frequently read data with groups that are sometimes requested via either 2nd or 3rd key specificity in the where clause, I would expect the multi-key covering index to perform better than included columns. Yes, that may be a contrived scenario but it might also be common for some shops.

How does the index with included columns compare to an indexed view? I guess storage space is of little concern if we're going to keep redundant copies of data in indexed views or included columns - but we commonly pay the price to trade space for speed, right?