504 supercedes both 502 & 504 AND is suitable for both flexible OCI & fixed OCI
Any real reason to use 504? Probably not. The TBN on 504 is almost half of 502 --- just something to consider.
There have been umpteen threads on this issue. Basically, 504 meets everything required by 502 and then more, and there is evidence (in the form of a published study available at the Lubrizol website) that the lower-SAPS LL03 504 oils result in far fewer intake deposits on direct injection engines.
So yes, if you have a DI car then there is a good reason to use a 504 oil.
In addition, TBN depletion is not linear, nor are the additives in 504 oils necessarily the same as 502 oils. If you're running something like a 5K mile fixed interval, it would seem very unlikely that TBN depletion would be an issue in a 504 oil.