I'm sure many viewers laughed when Parvati suggested that the sit-out bench should be named after Sandra. Those viewers are so focused on physical strength that they forget or don't understand the very nature of this game. Sandra herself wasn't irritated at all by Parvati's suggestion. In fact it amused her and she embraced it as a badge of honor. Sandra is right: While strength can help to survive, intellect and social skills have always been much more important. By accepting to sit out, Sandra not only helps her team but she avoids creating frustration in others that would be disappointed if forced to sit out. Being one of the eldest women and not a muscle-man, Sandra cannot be expected to dominate physical challenges.
If Survivor respected its own rules and prevented players from sitting out two challenges in a row then Sandra couldn't simply volunteer all the time. That rule, we realized, only applies for two challenges in the same episode but why not enforce it even when they have only one challenge per cycle? If they don't then we can't blame Sandra from profiting from the loop-hole.
The French Survivor Koh-Lanta, avoids this situation by having chance decide who will sit-out. If a tribe has an extra female then the women of the tribe pick rocks and the one picking the different rock sits out. Koh-Lanta's problem is that the same woman can sit-out if they are unlucky. I'd suggest Survivor use a variation of that method with the person who sat out the previous challenge being left out of the draw.