IMHO a leakdown is done if compression is suspect. I.e. the dynamic compression test shows that someting is not right. The leak down is also good on an engine that does not turn over / has been removed.
I bought my 993 with only dynamic compression test numbers. PO had a compression test done during the bigger services and the numbers were even between cylinders and consistent over the years. Gave me enough confidence.
There may be other situations where you may want to be more careful. Say, if I bought a modified 600 horse twin turbo, I would get a leak down.
Good Luck,
George