The only advantages silk has over rayon are: 1) it is stonger, so it hold up better when fished; 2) it lays flatter, so it is smoother when tied in; 3) it doesn't fray as easily as rayon; and 4) it can easily be split into a smaller size by poking a bodkin into it, rayon doesn't do to well with this. Rayon is a lot cheaper than good silk and more readily available. Since it works fine as tag material, use it (although I prefer to use high quality and unfortunately expensive Japanese silk).
A few other alternatives to silk that are readily available and in fact work better than silk for some applications are: 1) Uni-Floss, a nylon/rayon combination that I use for some Ally's Shrimp colors because it is single-strand and can be tied with a bobbin; 2) nylon florescent floss; 3) Uni-Stretch, a stretchable nylon that lays nice a flat when stretched, it is my favorite for Glasso Speys (it is easy to split to use in Glasso's split floss dubbing technique) and florescent tags (it is also very good for Ally's Shrimp bodies because it lays flat and is not very bulky.)
Heck, the old masters for the 19th century used substitutes for stuff when they didn't have a particular material, so why can't we?