You know Rosetl, your question is one I think about a lot too. I've come to the conclusion that there is no "best answer". There is often more than one good solution to a problem.
To me, the real advantage to mounting a textile against fabric, is that fabric has more of a texture/grip than matboard, so there is more overall support. For a fragile or very fine textile this can be a plus. There is also less fussing with hole punching through the matboard, and so maybe less stress on the textile during the mounting process. You can also "lighten up" a dirty textile with a light fabric underlay. Although I suppose you could do this with a light matboard too.
I was mulling this over in an earlier thread, and a very good reply in favor of matboard was that you guys have all the colored matboard samples to hand, and so don't have to go running all over town looking for the perfect color for the backing fabric.
I don't think waterfastness is much of a problem, as long as one takes care choosing (and washing)the backing fabric.
I think a lot of it boils down to how one was first trained to mount textiles, what materials one is used to working with, expediency, and time/money issues.
In the long run, the most important things are good quality, water-fast materials, no tape/adhesives, good overall support with no undue stress on the textile, and sealed protection from the outside air (Coroplast filler, or Mylar/Marvelseal wrap), and dim display lighting. Also, when the edges of the textile must be wrapped around a backing, I like to pad the edge (we've all seen those sharp turns/fold develop into tears!)
Myself, I prefer the fabric against fabric methods -for the above mentioned reasons and, it just seems "cosier"
! Open backed strainers or stretchers do need an archival quality backboard behind them though, to protect against polluted air flow and bangs/bumps.
In all honesty though, I've never seen a problem with textiles sewn directly to archival quality matboards (emphasis on archival quality!). Has anyone else?
Rebecca