I see no real point. 5.6 is too slow for a prime.
They're popular with the birds-in-flight crowd where there is plenty of light so f5.6 is fine. In that situation you usually don't need IS either though.The minimum focus distance is 3.5m... which could also do with an improvement but again, it's not really needed for BIF.
There are faster 400mm primes but they are very expensive compared to the 5.6