Parry...

[LoganNinefingers]

Um no to everything you just said in the beginning of your post, nothing in this game has diminishing returns. Every single test ever run confirms this. The exception being Speed, and ACC for different reasons.
Crit being the only stat that might have an argument for diminishing returns, even though I still have not heard a good one.

By process of elimination, what you've effectively said here is "of the other secondary stats that matter to non-mages, determination doesn't have diminishing returns, but the others do/might under certain conditions", so why the immediate "Um no" response?

The vast majority of testing I've seen into any of the less understood stats has been carried out by people who either

A) have very little working knowledge of statistics;
B) are lazy; or
C) both of the above,

as, in general, they don't quote all relevant data (usually just something along the lines of "15 points Crit Rate Stat gives 1% actual crit rate increase, parser said so"), and often only use two data points (with which it is impossible to infer anything other than an often misleading linear trend).
As such, a blanket statement like yours cannot be made with any certainty at this point in time.

The fact that both Menae and ZDamned (amongst others) have recently asked for clarification on whether there are diminishing returns or a "soft cap" for Parry shows that this issue definitely hasn't been concluded once and for all, and further speculation and testing is required.

So please, stop making these sweeping declarations before any real supporting evidence is in place; for now it just spreads ignorance around the player-base, and needs to be kept in check until we have the data to say for sure one way or the other.