http://www.sacredduty.net/2014/11/12...ds-of-draenor/
Refer to the part about stat weights and notice how stamina barely edges out mastery point-for-point. In aeons past (i.e MoP, where we still didn't give a flying fuck about stamina apart from very, very early and undergeared progression) stamina outperformed our secondaries by a ratio of 2:1 with regards to this metric and we didn't gem for it, ever.
Basically, by gemming stamina, you're retard-proofing your performance (in the sense that you're giving yourself more margin to fuck up) at the cost of a metric ton of DPS. Just using that stam trinket instead of a BA one is approximately a 3% AP loss, and doing silly stuff like gemming stamina on top of that is just making you even more of an unnecessary burden on everyone who has to slug through content behind you.
The only situation in WoD where using stam trinkets and gemming for stam doesn't make you a laughing stock is when all the damage from a fight is magical. That is literally the only situation in which stam trinkets are seriously acceptable over double BA or str trinkets.