The plastic bags I use are generally opaque pink, so they hide the fact that the saddle is a Brooks, which might deter a saddle theft.
One of my objections to the Brooks saddle cover is not that it advertises there's a
Brooks saddle inside. I just don't believe most bicycle thieves even know what a Brooks saddle is. My objection to the Brooks saddle cover is that it advertises that inside it there's a saddle worth announcing to those less fortunate. That's what makes it a thief magnet.
Not looking nice is actually a positive feature for me...
I dislike clothes that advertise the designer's name on the outside and avoid them where possible, which is pretty difficult for a cyclist. That Brooks saddle cover is a particularly ugly (in both manners and typography) example of advertising obnoxiously ostentatious personal expenditure. Both as a typographer and as an artist sensitive to such nuances, I cringed every time I came up to mine. You can't even turn it inside out, like wearing the whitewalls on your VW turned to the inside, because the wax coating makes it a one-way defender against rain. (And don't think that modern materials are any better. Goretex for instance depends on the nap of the nylon to the outside to shrug off raindrops because once the nap is saturated Goretex doesn't work any more. Goretex itself is not the entire thickness of the material but a micro-thin layer of one-way permeable gunge on the inside of what without it is basically a common nylon jacket. The mountaineer's trick of "restoring the Goretex" by washing the jacket and letting it dry in a draft to fluff up the nap, is actually the much less sexy "restoring the common overpriced nylon jacket so Goretex can claim the credit".)
Furthermore, while the Brooks cover may be less inconvenient for a tourer putting 200 miles into his day and sitting on his bike for hours, protecting the saddle from wet with his body, for a utility expedition or even social outings into the countryside with friends who will stop at every view and other excuse, a Brooks rain cover is a nuisance to put on and take off every hundred yards. I found I was walking when I could have ridden (already a crime I blame on Brooks) and leaving my bike exposed for longer than I am comfortable with even in my low-crime area.
Fortunately, by the time I discovered on this very cornucopia of esoteric knowledge that one isn't supposed to ride seated on the Brooks cover even for a short distance, I had conclusively ruined mine, so I wasn't stuck with an artefact I hated for it's snobbishness and counterproductive tendencies.
A further advantage of the shower cap faux Brooks saddle cover is that the thing is self-supporting: no fiddly, flapping, untidy velcro straps, no wondering what damage the wet cap will do to expensive photographic and sketching gear in the bags on your bike, you just shake the shower cap and twist it into a loose rope, and tie it to a convenient tube or for rail or bar -- or give it a quick swipe with your sleeve to knock the drops off and sit on it for the hundred yards between your pharmacy and your bank. It costs pennies to replace, and it doesn't try to prove how superior you are.
Salute the shower cap!