that's not really true (though it may be arguing meaningless difference)
in your example, you have 10% chance to block (90% hits resulting unmitigated) then 5% chance to parry the remaining hits, which results in 4.5% effective parry rate. While it doesn't matter for low numbers, it can result in a noticeable difference later on (30% block and 35% parry results in only 24.5% effective parry rate, 54% mitigation on hits when you could expect 65%). That's the main reason why a parry build is more effective on a WAR than a PLD considering raw numbers.
Now does it really matters ? Well, not that much. After all, you're gaining more from blocking than you're losing on the alternative (parrying) so overall your mitigation improves, and is better than the one of a WAR.
general consensus is "Don't stack it but don't refuse it either".
Parry > skill speed
