Well the good thing is you can work the single legs for a while without "sacrifising" anything on the actual supported single leg stuff because the pistols will just make you better at them. Whereas I haven't found that the supported stuff necessarily make you that much better at pistols..so if you think about it in terms of progression it makes perfect sense to just go ahead and rock the singles.
Plus the benefits as to mobility AND stability at the hips and ankles are way beyond any supported stuff.
BTW, that guy Steve Cotter can do like 15 or 20 of them holding 88 pound kettleballs in his hands...now that is strong, I don't care who you are