I'm not sure either... I've been using a bashring and jump stop all year with a medium cage X9 mech, unramped Hope chainring and the chain shortened. I haven't lost the chain yet. Maybe that's because I'm on a hardtail (no loss of tension under rear sus compression), maybe it's because I don't tend to slow down for rough stuff (it's always smoother with speed, pumping and skimming over the bumps)?
Maybe it's having relatively short chainstays, so when I'm going fast (and thus in the smaller rear sprockets) the chain angle means that the bashring is acting as both upper and lower guide? Even with longer chainstays a properly positioned bashring and unramped chainring should hang onto the lower chain run pretty well, assuming your mech isn't overly long, its tension spring too soft or your chain too long.