you def don't want to go higher if you can help it. pvp gear is crammed with versa so you will go above 30% without trying.

DR between 30% and 40% is only 0.1, so you're gaining 0.9%/0.45% instead of 1%/0.5% - and the DR is additive so it actually becomes better the more you get. If your class is a frequent focus target and doesn't reliably survive, versatility is absolutely still worth stacking after 30%. No other stat provides a mitigation bonus in addition to a throughput bonus.

If e.g. you have two moonkins hitting each other, the one stacking versa even beyond 30% will win against the one stacking any other stat, because getting 0.45% DR is more impactful than the 0.1% damage you lose to DRs.

versa is already the worst stat for damage while the defensive portion of it is only half the offense it provides, so getting less for each point is not good.

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ww monk is the only class where versa is actually the best offensive stat(and not the worst by far like other classes) so they can actually stack it a bit to the point where it's worth the same as other stats with a bonus to defense still.

Versa isn't the worst stat for damage. Since crits are nerfed to 75% extra damage in PvP, crit is automatically worse than versa for like 90% of all specs, even excluding the damage reduction for versa. Even with the 10% DR, it's still way better than crit which is effectively reduced by 25% at all times in PvP.

For many specs, mastery or haste can also be worse for throughput than versa, especially during burst since some specs' burst abilities aren't affected by one of the two, e.g., haste for abilities like strike of the windlord, arcane surge, odyn's fury, etc.

some people still like to stack it, but it's an absolute fact that a portion of your stat points are vanishing into thin air when you do this.

So are they if you pick suboptimal secondary stats. Any spec without crit bonuses picking crit over versatility is just griefing themselves and throwing away free damage reduction.