[SFfiredog]

There are theoretical advantages to internally balancing a rotating assembly. Idealy, one would like to have as much of the balancing weight in the center counterweights of the crankshaft as possible.
Sustained high RPM engines sometimes will have one or two extra center counterweights to help spread out the load on the crankshaft. This, of course, adds weight to the rotating assembly that would not be desirable on a road race or drag race engine.
The main advantage, and the only real reason to internally balance a normally externally balanced engine is the rotating assembly is not dependant on the flywheel or dampener being matched to the engine at the time the balancing is done.
Since these parts are dynamically balanced to themselves one can change dampeners or flywheels on a internally balanced engine without throwing off the balance weight.
In my opinion, unless you have an engine you plan to spin over 7500 to 8000 rpm (small block) or 6500 to 7000 (big block)regularly or want to swap lighter or heavier flywheels at any time the added expense of internally balancing would be better spent towards upgrading some other internal component (lighter custom pistons, aftermarket rods, etc.)
HTH,
--Mike