Why 5-ply?

Alden Amos says in his, Big Book of Handspinning, that there is no reason to spin 5-ply yarns.

In fact, 5-ply yarns are more supple for their thickness, and thus easier to knit into weatherproof fabrics.

5-ply yarns are also more durable than a 2 or 3-ply yarn of the same fiber and grist.

Those issues may not enter into your decision making, but if you are thinking about knitting a sweater that will keep a seaman warm on a long sea voyage, they are valid reasons.