Your concerns about pistons and beach entries is spot on! The grit that adheres to the fine crevice where the piston oring meets the land accelerates the deterioration of that design even with "thorough rinsing".
The same concerns do not really apply to the "unsealed" Mk11. The diaphragm seal protects the innards completely, as long as you are careful about salt water droplets at the intake filter when you change tanks.
The outside of the diaphragm and the mainspring are easily flushed with water, no matter how sandy they might get. If you value the compact design, then that's enough reason to forego the extra environmental sealing chamber. For many divers that chamber often becomes a small problem, because the seal of the second diaphragm is quite light, and shops commonly seal it incorrectly, with the reg depressurized, so it ends up bulging. This causes the first stage IP change with depth to lag, until ambient pressure with depth pushes the second diaphragm flat against the plastic piston that transmits pressure to the main diaphragm.
One more complication, unless you're familiar with reg repair. Me? I like that seal, but it's not critical, even in sandy conditions.
