I'll weigh in.
Sure, Dale drove different numbered cars other than the 3. Dale Earnhardt was the face of NASCAR when he drove the 3. Richard Petty was the face of NASCAR in the 43. Although both drivers raced against each other, they were not dominant at the same time. Dale pretty much took over after Richard's reign.
That being said, when you think of the black 3, you think of Dale Sr.
When you think of the 43 (STP, Petty Blue, etc.), you think of Richard Petty.
You will never associate any other driver with either of those two car numbers, and you shouldn't. Both of those drivers were so influential to the sport of NASCAR, that I feel the numbers should be retired to preserve their history in the sport of stock car racing.
There will never be another Richard Petty.
There will never be another Dale Earnhardt.
They should retire the 3 and the 43. JMO.