With Intercontinental Hotels, there are some situations where a reservation becomes non-viewable online. But at least you can see the reservation in the summary list: so you can still see the hotel name, intended stay dates and confirmation number.

With Starwood, some reservations disappear, or never appear in the first place, even though agents can see them when I give them my SPG number.

I just know that sooner or later I am going to have a reservation I no longer need but forget to cancel because I don't see it on "My Stays".

Another concern would be if human error or fraud resulted in someone else using my SPG number to book an award stay. Being unable to view all reservations booked against my account significantly diminishes my ability to protect myself from such a situation.