Greetings Jim :-)
in reply to Modification of @ISA at run time
I guess one reason might be the example given below
Suppose your whole application has a few lakhs lines of code and If some body has to maintain this , It will be easier for them if they can figure out by looking at the class (by looking at use base), If there are such runtime heirarchies It would be very difficult for them to figure out the heirarchies
If they have to create a new class which uses your class as a base class, they will have to know the Inner workings of your class ..