Normally the Spartans were very much concerned about helots uprisings, and there were indeed several large recorded ones. On the other hand, when needed (when pressed really hard), they would draft helots to their army, and usually they granted them freedom for this. Several such instances are described by Thucydides and Xenophon. There is indeed no recorded case that I know when those liberated and drafted to the army turned their weapons against the Spartans. But why should they? To achieve what? They were already liberated. You may argue that the purpose would be to liberate other helots,
but this is apparently not the way they thought.
Of course these liberated helots did not become real Spartans with full rights,
but they became personally free, probably similar to ''perioikoi'' (which constituted a large part of the population). But this was similar to the situation in other city states: not all inhabitants had full citizen rights.
By the way, Athens also drafted slaves to their navy, when pressed hard.