Is there any reason to need a Manyullyn pickaxe?

Manyullyn pickaxes have a vastly superior durability factor. With Tinker's Construct, Manyullyn is very nice for weaponry, but in this case, it's one of the best pickaxe head modifiers in the game.

It gives you the best mining level, fastest mining speed, and highest durability. You should combine it with a Slime handle and an obsidian binding, and then modify it with redstone, a mossball, a gold block, and diamond. This will provide one of the best pickaxes available.

TL;DR: Manyullyn is good for pickaxes because while it mines slightly slower than cobalt, it has a far higher durability, and can be modified to mine just as fast as cobalt.

In response to: "aside from a durability increase" from the OP:

Durability is extremely important and is basically the only reason for using Manyullyn instead of Cobalt.

If durability does not matter to you, just stick with Cobalt.