Those may stack, but they don't multiply on each other like defense bonuses do. You're also forgetting the biggest thing, which is that that 7,5% defense to all, lets you REALLY ease up the build more where that defense can be used, allowing for you to slot more bonuses and procs to alleviate the build. Not to also mention, that that extra crit from the ato, and eagles claw, will work on every other attack you do, whereas that 5% extra crit on just storm kick alone ONLY works for storm kick. Its really NOT a contest to which is better, that 7.5% defense bonus would hands down help out any build better than 5% more crit on just storm kick would do.
And for the record, as you just stated, my martial arts scrapper IS ma/nin, past soft cap, and i would still much rather have that 7.5% defense to loosen the build rather than a crappy 5% more crit on just one attack, ITS NOT EVEN CLOSE.