Actually,
They should use COALESCE, it's much easier to use, doesn't request nesting for multiple tests and is ANSI SQL compliant.
ISNULL may not be ANSI SQL compliant, but I think you will find it faster than COALESCE when you only need to test a single value for a null value. Using COALESCE makes sense when you have multiple values and you want the first non-null value.
To throw another option into the mix for those that may not know about it:
You could use IS NULL (or IS NOT NULL for the opposite) in the where clause. Depends on the question that the SQL is trying to answer.
Good ol' SQL, "It Depends" seems to crop up a lot doesn't it 🙂
-d