Sporran straps

[Nanook]

That is a perfect solution and, let's face it, anything is better than having a metallic chain constantly rubbing against your expensive woollen kilt.

The acute problem is less, I think, the use of metal in chains but the use of cheaper grade chains. A well designed chain should not have any edges to catch fibres and should be quite smooth. Leather, and especially the "buff" rear, by contrast is less than slick. Relatively smooth leather belts are used to polish (sharpen) knife edges. In theory, at least, it seems a properly made chain should rub less than a belt.
So what can wrong with chains for daily use? The problem is really abrasion from having a sporran (held on by whatever means). Its the same effect that wears out the seat of our finest woolen pants, the edges of the cuffs of coats and shirts and in overcoats the edges of hand pockets (used to warm one's hands). The key to the claimed effectiveness of a sporran strap in showing less wear I would suspect is more derived from its restricted movement (less swing, tighter) than from the use of leather. Regimental straps are, in fact, made of a rough woven (to imitate from a distance buff leather) but very rugged thermoplastic polymer: Courlene. Its the tassels (and, in the case of horse hair, long hair) that swing. Even for ceremonial use one uses straps since the whole sporran must sometimes (often) be moved to the hip.
Chains are typically a bit looser and the sporran is let to swing a bit. It allows, I think, for more freedom of movement as well as centered placement. That's the advantage in a formal setting. Great for standing with a glass in hand or sitting but not the ideal for "marching".