I guess it comes down to:
cleaning the old condenser is a pain, and you have to get all the old mineral oil out when converting to r134a, since r12 is stoopid expensive stateside.
I havent seen one OEM condenser that is not damaged, ie bent fins, corrosion etc, all which reduce its efficiency...
since r134a is much cheaper, it makes sense to convert from r12. the parallel flow condensers are more efficient for use with r134a, than the old serpentine single pass.
Oring fittings are much better at sealing when used with r134a. getting rid of as many flare fittings as possible reduces the chance of leaks.(not as much an issue with r12 since the molecules are something like 100 times larger and generally much less leak prone)
so, in a nut shell, if you are converting to r134a, its almost a must. If staying with r12, and the OEM one is in good shape, keep it.