The securable views are pretty useful, but suffer from a couple of important limitations.
The first is that each view will contain every record from its source table. This isn't always desirable - it may be ideal to create a view of a fact that contains only certain kinds of transactions, or a view of a Type II SCD dimension with only the current values. Right now, the only way to accomplish this is to create physical versions of these tables, which is a clunky solution at best and unviable at worst.
The other problem is that you can't control what columns end up in these views. The most obvious problem issue is that this means that all of the system fields will show up in the securable view, which is not desirable because these views are almost always exposed directly to the user. It also reveals the existence of all the columns in the table to everyone who has access to the view, even if access to those columns is restricted by a security policy.
If these two features were added, securable views would be considerably more flexible and powerful tools!