Nurgle Havocs are better than many would think, but my opinion is based off of my experience with Death Guard rather than Black Legion (death Guard lack the versatility, which is the only reason I say it).
Give them Flamers (I run a unit with 3 flamers, though if I took my army to a higher points limit I'd take it up to 4) and set them up fairly close to the enemy. If they're running an assualt force you can put 'em right up front, otherwise screen them with good assualt troops who you expect to bite the dust before they reach the front lines. Then you get some flamers headed their way, as well as the Havoc Aspiring Champion, which should be fairly tooled (I go with Plaguesword, Daemonic Strength and Nurgle's Rot, though in BL Rot isn't so good, and could be swapped for Mutation).
It's a nice way to give your force some variety without costing too many points, not to mention a pretty resilient unit. If your army is led by a Khornate devotee, your Tzeentchian and Nurglesque units will all be elites (make sure you don't go overboard), just don't have second thoughts and take a Tzeentchian leader, as the havocs would go down the tubes.
I wouldn't highly recommend them, but in my experience they're fun (especially when they're actually useful, like in cityfight or when assualting an outpost where the opponent thinks that cover matters), and if you've got points lying around it's not a bad investment. You might want to proxy them and see if you like them before getting the models though.
