Well I have had the "almost failed the roll" roll =)
If water gets into the kayak while upside down it can really throw off your balance.
This past weekend I did a upside down re-entry and roll in the pool. I got it, but almost rolled right back the way I came.
You're right there really is no such thing as a "bomb proof" roll. However it is essential if you don't want to swim to practice it and practice it even more.
ACA Level 4 Kayak Instructor
Wilderness Systems Tempest 170