  We study the extent of the reversible region in the vortex phase diagram of
recently available RbOs$_2$O$_6$ single crystals \cite{Rogacki08a} by means of
bulk magnetization measurements. We found that the irreversible magnetic
response sets in at a field $H_{\rm irr}(T) \sim 0.3 H_{\rm c2}(T)$ for $0.5
\lesssim T/T_{\rm c} \lesssim 0.8$, yielding a reversible vortex region that is
uncommonly wide in comparison with typical low-$T_{\rm c}$ materials. The
relevance of thermal fluctuations is limited since we estimate a Ginzburg
number $G_{\rm i} = 5 \times 10^{-7}$. However, the relevance of quenched
disorder is unconventionally low since the critical-current density ratio at
low fields and temperatures is of the order of that found in high-T$_{\rm
c}$'s. We therefore conclude that an intrinsically low bulk pinning magnitude
favors the existence of an unexpectedly wide reversible vortex region that
seems to be generic to $\beta$-pyrochlores.
