If you are looking for flexible heavy weapon support take obliterators. (They are more flexible then ML's, and they are quite a bit tougher to boot!)
If you are looking for anti-air use heldrakes or aegis defense line. (heldrakes because DUH, and aegis because interceptor +hard to remove+nice cover as a bonus)
5 Nurgle Havocs with ML's are just going to be mediocre at everything for their points cost especially with Flakk.
This is only if you want to "min-max". If you want to take the Havocs then take em.